Assuming the expression is equal to zero, cos(theta)-5sin(theta)=0, cos(theta)=5sin(theta), so divide through by cos(theta) and we get 5tan(theta)=1 so tan(theta)=1/5=0.2 and theta=11.31° approximately.
There are other answers: 180+11.31=191.31°, and so on, adding or subtracting 180 degrees each time.