Q is directly proportional to the square of r. q=3 when r=4?

q = kr^2 where k is not zero.

When r = 4, q = 3. This means
3 = k(4)^2
k = 3/16

Therefore, q = 3/16 * r^2.

First find k

Q = kr²

3 = k 4²

3 = 16K

k = 3/16

k = 0.1875

Formula now is Q = 0.1875r²

q = kr^2
when r = 4, q = 3
3 = k*16
k = 3/16