?*Solve*

Solve[{x + y == 5*a, x - 3*y == 3*b}, {x, y}]

Solve[{x + y == 5*a, x - 3*y == 3*b, a + b == 4}, {x, 
        y}, b]

Solve[Exp[a*x] + (x/a) == 1, x]