Math and science are used together because when you are doing science you sometimes need to use math to help you with what you are learning. For example in class we were doing this activity where we learn what length of a mirror we will have to buy so that we are able to see ourselves completely. This activity showed us how reflection of the mirror makes you bigger or smaller when you get closer  or farther. We used math in this activity to graph the activity that we doing. When we graphed the activity we had to calculate a ratio to see what kind of size mirror we will have to buy if we were to buy a mirror. With our calculations we found out that if we were to buy a mirror that we could see ourselves from top to bottom the mirror will have to be half our size. That is what math can help learn when you use it in science.
When do we see negative numbers? We would of course see them on calculators, math class, and maybe even on math topic restaurants but the place where you would see them is when you solve a problem like what is 2-4 and the answer would be of course negative 2. That's way that you solve that problem is by first learning the number line including the negative numbers. So for example if right now I were looking at a number line that goes form -4 to 4 I would see it like this: -4, -3, -2, -1, 0, 1, 2, 3, 4, and of course it continues.
Negative numbers are very hard to find in everyday places because how many times a year are you going to run into a store or person or anything like that, that is going to have a bunch of negative numbers. Most of the time negative numbers are usually in math problems that can help you with everyday things. Some of the everyday things that negative numbers can solve for you in science and many of other things.
The first step you will have to do to solve the problem 2x-7=15 is by adding 7 to 15. Think of this, we are working our self backwords in the problem and the last thing that we would do in the problem is minus the seven. So lets continue with the equation. Next we will have to find how much times 2 can go into the sum of 7+15 which would be the answer 11. the answer to the problem is 11. To check if the answer to this problem is correct solve the problem again but this time adding in the number replacement of the variable(in other words the answer you got with the problem you just solved). This is how you would solve equations similar to this one.