x^2/a^2=1+y^2/b^2=(b^2+y^2)/b^2; a^2/x^2=b^2/(b^2+y^2); a^2=x^2b^2/(b^2+y^2); a=+xb/sqrt(b^2+y^2). The quadratic equation has no a term, only an a^2 term, so the formula isn't necessary and the solution simplifies to involve only the constant term, which in this case involves b, x and y.