DSolve[-y[x]^2 + a^2*Sqrt[x^2 + y[x]^2] + 2*x*y[x]*Derivative[1][y][x] + (-x^2 + a^2*Sqrt[x^2 + y[x]^2])*Derivative[1][y][x]^2 == 0,y[x],x]