Jack bought 3 slices of cheese pizza and 4 slices of nushroom pizza for a total cost of $12.50. grace bought 3 slices of cheese pizza and 2 slices of mushroom pizza for a total cost of $8.50. what is the cost of one myshroom pizza

The cost of one slice of mushroom pizza is $2.00 dollars. This is solved by subtracting the equations:  
X = cheese pizza 
Y = mushroom pizza  
(3X + 4Y = 12.50) 
- (3X + 2Y = 8.50)
  
2Y = 4.00 
Y=2 or a slice of mushroom pizza is $2.00.