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