The equations will look like this:
(1) C + P = 264
(2) $2.5 * C + $3.25 * P = $810
Calculate from (1):
C + P = 264
P = 264 - C
Lets put P into (2):
$2.5 * C + $3.25 * P = $810
$2.5 * C + $3.25 * (264 - C) = $810
$2.5 * C + $858 - $3.25 * C = $810
$2.5 * C - $3.25 * C = $810 - $858
- $0.75 * C = - $48
C = 64
P = 264 - C
P = 264 - 64
P = 200
Pizzeria sold 200 slices of pepperoni pizza and 64 of cheese ones.