arccos -0.327....
=arccos 0.327..
If you can make a sketch, this would help you a lot.
Let's call the force 115 lb being along a vector OA and the 215 lb force along a vector OB. Lastly, OC being the vector along the resultant 275 lb force.
using the cosine rule, we can get the angle OBC.
Now, if you have your sketch, you'll know that OACB is a parallelogram. You are looking for angle AOB, the angle between the two forces. You have OBC and you know that angle OBC = angle OAC and angle AOB = angle ACB. Then, we know that in a quadrilateral, the sum of angles is 360 degrees. You can find angle AOB now.
I hoped it helped!