See figure attached.
I tried to split all the forces and calculate the perpendicular distances from the point E. I did this for the 2 forces in the 2nd and 4th quadrant but I still don't have enough information to determine alpha.
EDIT: Update, I think I managed to solve it by simply expressing the perpendicular distances of the 130N force in terms of alpha.
Anyone care to check my answer?
I got a value of 39.1 degrees for alpha.