Solve[a*x^2 + b*x + c == 0, x]

Solve[a*x^3 + b*x^2 + c*x + d == 0, x]

Solve[{1*x + 2*y == 3, 3*x - 5*y == -7}, {x, 
        y}]

Solve[{a*x + b*y == e, c*x + d*y == f}, {x, y}]

Solve[{a1 == a2, T1 - m1*g == m1*a1, 
        T2 - m2*g == -m2*a2, T1 - T2 == 0}, {T1, T2, a1, 
        a2}]

Solve[{2*a1 == a2, T1 - m1*g == m1*a1, 
        T2 - m2*g == -m2*a2, T1 - 2*T2 == 0}, {T1, T2, a1, 
        a2}]

Simplify[
      Solve[{N*Sin[theta] == m*ax, N*Cos[theta] - m*g == m*ay, 
          ax - A == -ay*Cot[theta]}, {N, ax, ay}]]