Hi

Parallelogram law states that if two vectors are considered to be the adjacent sides of a Parallelogram, then the resultant of two vectors is given by the vector which is a diagonal passing through the point of contact of two vectors.

According to parallelogram law of vectors the resultant is represented by the diagonal  passing through the point of contact of two vectors.

R2=(P+Qcosθ)2+(Qsinθ)2=P2+Q2+2PQcosθ

Resultant acts in the direction making an angle α=tan −1 (Q sin θ/ P + Q cosθ ) with direction of vector P .