A dozen more than a number minus twice the number

The line containing the midpoints of the legs of right triangle abc where a(-5,5), b(1, 1), and c(3, 4) are the vertices.

Which simplifys to a+ b? a-(1-b)+1 -(1-a)-b+1 (a--1) -1)+1

Asequence is defined recursively using the formula f(n + 1) =f(n) - 5. which sequence could be