Why is it appropriate to use the trigonometric functions on triangle FGJ but not ontriangle FHJ?

The trigonometric functions are real functions which relate an angle of a right-angled triangle to ratios of two side lengths.
In the given, ΔFGJ is a right triangle, as it has a 90° angle. ΔFHJ on the other hand is not defined as a right triangle, and thus we cannot use the trigonometric functions on this triangle.