How do i get from the following Recurrence Relation
$T(0) = 0$;
$T(1) = 7$;
$T(n) = T(n-2) +7$ for $n$ $\ge$ 2
to the closed form? I know that the solution is
$\frac{7n}{2}-\frac{7(-1)^n}{4}+\frac{7}{4}$
But my problem is getting there, for other recurrence relations i can use the arithmetic series or geometric series to get the closed form but i don't know how I could use any of them here, I tried to insert some small integers for n and got the Sequence
$0, 7, 7, 14, 14, 21, 21$
$T(2) = 7, T(3) = 14, T(4) = 14, T(5) = 21, T(6) = 21$
but that doesn't help me at all to be honest, is there any tip or trick to get easier/faster to the closed form? Any help is appreciated.