Dear Noora7939, the pair of planets that exerts the strongest gravitational force is Saturn and Jupiter, since we know that both Jupiters are the same mass, we'll excuse that. Since the laws of gravity says that the bigger the mass the stronger the gravitational pull, Saturn has the stronger gravitational pull since Saturn is bigger than Mars.
5 (7) - 3 (-8)
35 - -24
Because two negatives equal a positive the subtraction sign will change into an addition sign.
35+24
So your answer will be 59.
Hope this helped :)
You have a lot of questions here, I can help you with a few of them.
When you simplify the polynomials, you put together all of the terms that have the same degree. That means the variables have the same exponents. The number of terms are the items separated by plus and minus signs.
Here are the answers for the first three:
1)

2)

3)