It is just a second order polynomial. (a^2)/b + a – x =0. The quadratic formula applies. The unknown is a, with 1/b for the coefficient of the square term, 1 for the linear term and – x as the constant term.
Simplify the calculation by multiplying both sides of the equation by b, assuming b is not 0.
a^^2 + ba – bx = 0. The quadratic formula is x = 1/(2a) (- b +/- sqrt(b^^2 – 4ac). In this case, substitute 1 for a, b for b and -bx for c.
Then you get the same answer as Wolframalpha. I don’t know why they broke the square root into two parts.
a = [ – b +/- sqrt(b^2 + 4bx)]/2