I have no idea how to do the f(37) function
f(0) is -3
f(1) is 0
f(5) is -6
f(37) is???
$37 = 6\cdot 6 + 1$
So, $f(37) = f(31+6) = f(31) = f(25+6) = f(25) = \cdots = f(1)$
You found out that $f(1) = 0$ above. So, $f(37) = 0$.