When you see the symbol "△", this denotes a triangle. Now combine the symbol with the letters FGH, this becomes △FGH, meaning a triangle FGH, with F, G, and H as the three points.
Now, the symbol "∠" means angle and "m∠" means the measure of an angle. So m∠F means the angle at the point F, m∠G means the angle at the point G, and m∠H means the angle at the point H. (I'll attach a diagram of this triangle).
Now we have to determine m∠H, which is the angle at point H. To do that, we have to find the value of . To do that, we have to use one of the laws of a triangle, which is:
The sum of angles in a triangle = 180°. This means when we add all the angles in a triangle, it sums up to 180°. Let's do that.
Now that we have found the value of x as 18°, we can substitute its value to determine the angle H.
I hope you understand the explanation.