I varies inversely with r+2 if there is a constant k such that

I = k/(r+2)

I = 9 when r=3 that is

9 = k/(3+2)

K = 9*(3+2)

k = 45

thus we have I = 45/(r+2)

when I = 8 we will have

45/(r+2) = 8

r+2 = 45/8

r = 45/8 - 2

r = 29/8

r = 3.625