How do i use the method of elimination?

In a right triangle δabc, the length of leg ac = 5 ft and the hypotenuse ab = 13 ft. find: b the length of the angle bisector of angle ∠a.

Given: m zade = 60° and m2cdf = (3x + 15) prove: x = 15

For what values of a is the sum of the fractions 2a−1 / 4 and a−1 /3 positive?