To find the related angles of given ones, and their (+) or (-) signs, use the unit circle.
cos150°= cos(180°- 30°)= -cos30°= -(√3)/2 (30°-60°-90°, or 1-2-√3 triangle), sin90°=1, and
tan210°= tan(180°+30°)= tan30°= 1/√3 = (√3)/3 (30°-60°-90° triangle)
The given equation is restated as follows:
2cos150°- 3sin90°+ tan210°= -2cos30°- 3sin90°+ tan30°
= -2(√3)/2 - 3(1) + (√3)/3 = -(9 + 2√3 )/3 (= approx. -4.1547)