Solve f'(x) = 0, this means which is a 2nd degree polynomial with a = 2 > 0, this means is negative between its roots [x1, x2] and positive for the rest (-inf, x1) U (x2, +inf), if a < 0 the sign would be reversed.

if f`(x) < 0 then f(x) is decreasing on that interval, f`(x) > 0 increasing on that interval

i forgot to mention is always positive so it doesn't interfere with the sign of your derivative