draw a diagram showing the Cartesian axis and your vectors.
Let be the vector that represents the force of 120 lbs
and be the vector representing the 200 lbs
then the resultant force is given by:
i suppose you will know how to draw these vectors.
let be the terminal point of
and be the terminal point of
so, we would have
once we have we can figure out what is required of us.
can you continue? do you even know what i am talking about?