first one is -30p = 0 because distribute 5 to get 60 - 15p and its the same on both sides except for one sign.
second has infinite solutions because distributing gives 3y - 6 = 6y - 6 - 3y. Combine like terms to get 3y - 6 = 3y - 6. When the equation has a number equaling another number there are infinite solutions.
Third one is a = -2 because if you add 4a to both sides you get 7a + 21 = 7. Then subtract 21 from both sides to get 7a = -14. then divide by 2 to get the answer.