Its simple really
To make A the subject of the equation r = sqrt(A) / N, just do this:
Multiply both sides of the equation by N: r * N = sqrt(A)
Square both sides of the equation: (r * N)^2 = A
Therefore, the equation with A as the subject is:
A = (r * N)^2
So, the answer is A = (r * N)^2.