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nbrion's answer's usefulness can be slightly enhanced with a few assumptions:

 sage: with assuming(b>-pi/2,b<pi/2): ((eqn2[0]).solve(b)[0]/cos(b)).trig_reduce()
....: .arctan().trig_expand()
b == arctan(z*cos(c)/x + y*sin(c)/x)

HTH,