The cost of a circular mirror is directly proportional to the square of the diameter.

A circular mirror with a diameter of 6cm costs $50.

What is the cost of a circular mirror with a diameter of 90 cm?

This is what I tried:

If a 60cm mirror costs $50, then each cm is 50/60 = 0.83 per cm. Therefore, 90 * 0.83 = 75 cm. But this is wrong. Help.

PS how do I put up a graph question here? There doesn't seem to be a copy an attach or copy & paste function.