With equations like these you're basically just plugging numbers in where there are variables. so if you have 5x-2x-x when x=5, you just plug in 5 wherever x is. 5x-2x-x turns into (5•5)-(2•5)-(1•5). you multiply the expressions in the parentheses to get 25-10-5 and then you simply subtract.
1. 5x-2x-x
2. (5•5)-(2•5)-(1•5)
3. 25-10-5
4. 10
We can do the same thing with your second example. 9z•7p when z=2 and p=3. When there is a z, you plug in a 2. When there is a p, you plug in a 3.
