5a + 3 - 3a = -7 how do i get the answer

What you want to do is move all the a's on one side, because that is what you are trying to solve. So it would be 2a+3=-7. Then you would move the numbers over, the ones with no a attached to them, so you would subtract 3. And it would look like this 2a=-10. Then divide by 2, and the answer is a=-5.