DSolve[f[x]^2*y[x]^4 + (2 + a)*f[x]*y[x]^2*Derivative[1][y][x] + a*y[x]^3*Derivative[1][y][x] - (-1 + a)*Derivative[1][y][x]^2 + a*y[x]*Derivative[2][y][x] == 0,y[x],x]