I assume by theta, you mean x.

Lets convert those degrees to radians first:

20degrees -> pi / 9 radians

180degrees -> pi radians

4(3^(1/2))sin(3x + pi/9) = 4cos(3x + pi/9) -> divide both by cos(3x + pi/9)

4(3^(1/2))tan(3x + pi/9) = 4 -> divide both by 4

(3^(1/2))tan(3x + pi/9) = 1 -> divide by root 3

tan(3x + pi/9) = 1 / (3^(1/2)) -> take inverse tan of both

3x + pi/9 = tan-1(1 / (3^(1/2)))... more