Answer:
a = -5
Explanation:
You have to isolate the "a", and since the -15 and the a are being multiplied together (you can tell this because they are beside eachother), you have to divide -15 by a. Then do the same for the 75. You do that to both sides because of the =, so in order for the sides to equal the same still, you have to do the same things for both of them.