Hi. The statement you gave is somewhat true, but not always. I disprove this and there are tons of websites that can help you with more in depth explanation. Have a good day.
Speed and velocity is the answer
-x^2 + 4x + 12 = -3x + 24
-> x^2 - 7x + 12 = 0
-> (x-3)(x-4) = 0
-> x= 3 or 4
so y = 15 when x = 3, y = 12 when x = 4
