Constructing a parallelogram which size is 6cm and 9cm the angle between size being 60 degree menion the diagonal
do you even look at what you type?
That's "sides" not "size"!
and "menion" just escapes me totally!
If the two diagonals are x and y, then using the law of cosines, and the fact that consecutive angles are supplementary,
x^2 = 6^2+9^2 - 2*6*9 cos 60°
y^2 = 6^2+9^2 - 2*6*9 cos 120°