Help me w/ this ty o_0

What is the length of the unknown leg in the right triangle ?

For the rule of 78, for a 12-month period, the last term in the sequence is 12 and the series sums to 78. for an 10 month period, the last term is and the series sum is . for a 15 month period, the last term is and the series sum is . for a 20 month period, the last term is and the series sum is

Solve: 3x+y=6 4x-2y=18 show all work !