When two are more forces act simultaneously or concurrently on ONE body, the body sums up these forces and moves as if there is one single force acting on it.
This single force is called the resultant force of the many forces acting on the body.
there are two forces P and Q acting on a body (through the center of gravity of the body, we assume). P & Q are vector quantities. The resultant force R is given by the parallelogram law of forces (or vectors).
magnitude of R² = P² + Q² + 2 P Q cos Ф where Ф is the angle between the two lines along which P & Q are acting. If P & Q are acting along one line , Ф = 0. If they are opposite to each other then Ф = 180.
This resultant R makes an angle α with the force P, then
tan α = Q sin Ф / (P + Q cos Ф)
If you draw a parallelogram with the magnitudes of P & Q as sides then the diagonal passing through their intersection is the resultant force. THe other diagonal (shorter one) is the resultant of forces P & - Q.