Make y the subject of these equations
a/y²+c=b
If you meant
a/y^2 + c = b
a/y^2 = b-c
a/(b-c) = y^2
y = ±√(a/(b-c))
Or, assuming the usual carelessness with parentheses, maybe you meant
a/(y^2+c) = b
a/b = y^2+c
y^2 = a/b - c
y = ±√(a/b - c)