What is the nth term 4 11 20 31 44 59?

The interval grows by 2 for each term so this is a geometric sequence involving a n² somewhere.
x0=4
x1=x0+7 x1=4+(5+2(1))
x2=x1+9 x2=4+(5+2(1))+(5+2(2))
x3=4+(5+2(1))+(5+2(2))+(5+2(3))
rearranging
x3=4+(5+5+5)+(2(1)+2(2)+2(3))
The end of this is the triangular sequence, so...
xn=4+5n+2(n2+n)/2
collecting terms and simplifying...
xn=4+5n+(n2+n)
xn=n2+6n+4 ■