y varies inversely with the square of x:
square of x is x2
Inverse of square of x is 1 / x2
So the equation would be:
y = k / x2
where k is a proportionality constant.
To find the value of k, plug in the known values for x and y: x=3, y=45
45 = k / 32 = k / 9
Now you have the full equation with proportionality factor.
y = 405 / x2
then, for x=5:
y = 405 / 52 = 405 / 25
y = 16.2