y is proportional to x^2. When y=45, x=3. Find a formula for y in terms of x.

If y is proportional to x then you can write y=kx^2 where k is a constant you are trying to find. Then substitute in the values you have been given for x and y. This gives you 45=k(3)^2. Then work to solve this to find k. 45=k(9)k=45/9. Therefore k=5. You can now put your value for k into the formula to give you an answer of y=5x^2.