Y is inversely proportional to the square of x

y  = k /x^2

We need to find k

And we know that

9 =  k / 1      multiply both sides by 1

9  = k

So......when y  =  16, we have

16  = 9 / x^2     rearrange as

x^2  =  9 /16        take the positive root

x  =  3 / 4