You are looking at the wrong triangle. You need to look at theforcetriangle, not the triangle formed by the slope. If you draw a picture, representing the slope as a right triangle with angle 18 and 72 degrees, the 72 degree angle being at the top of the triangle. But you divide theforceinto components, not the slope. The gravitational force, 30g, is downward and that forms thehypotenuseof the force triangle, while the component along the slope, the force friction must be equal to in order that the crate not slide, is thenear sidein this triangle: friction force/30g = cos(72), not the other way around.

Thanks in advance