help please!!!!!!!!!!!!!!

1. You can make use of the Pythagorean theorem considering "a" to be the longer leg of the largest right triangle. Then the value of "a" can be calculated from
(16+9)² = 15² +a²
625 -225 = a²
√400 = a = 20
2. You can find the altitude of the largest triangle (the length of the unmarked vertical line), then find "a" as the hypotenuse of the medium-sized right triangle.
That altitude is ...
√(15² -9²) = √144 = 12
so the length "a" is ...
a² = 12² +16² = 144 +256 = 400
a = √400 = 20
3. Based on the 15 and 9 dimensions of the smallest right triangle, you can realize that these triangles are multiples of the 3-4-5 right triangle. Then you can use relevant ratios of the side lengths of any of the triangles of which "a" is a part.
a = 4/3×15 = 4/5×(9+16) = 5/4×16 = 20