y is proportional to x^2. When y is 75, x is 5. Find a formula for y in terms of x

y is proportional to x² so this means y = A*x² for some constant A.

When y = 75, x = 5. Put these values in the formula above to find A:

75 = A*25

75/25 = A

A = 3

So y = 3x²