Vector Problem - Angle between two vectors

[jaredm2012]

Homework Statement

Vectors A-> and B-> have equal magnitudes of 55.0. If the sum of A-> and B-> is the vector 17.7 j, determine the angle between A-> and B->

Homework Equations

cos(theta) = {[A]^2 + ^2 - [A-B]^2}/2[A]

(sorry, not sure how to type out that formula!)

tan(theta) = (Ay + By)/(Ax + Bx)

R =[(Ax + Bx)^2 + (Ay + By)^2]^1/2

The Attempt at a Solution

This one I am not sure about how to start, at all. I plugged what I could into those formulas, but any answers I got were incorrect. I don't understand how I can find out any more information with what is given, namely the other components of the vector (the i for each, as well as discerning which values for j each vector has).

 

[rl.bhat]

Since resultant is 17.7j, it is along y-axis. So its magnitude is 17.7. Magnitude of A ans B 55.0 each.
Use the formula R = [ A^2 + B^2 + 2ABcos(theta)]^1/2 and find theta.