Stephen buys x pencils and Y notebooks for his students. Each pencil costs $0.45 and each notebook costs $1.25 He buys a total of 210 pencils and notebooks for $142.50.
How many pencils does Stephen buy?

Iam unsure about what the proof is, therefore as much as wish to i can not.

You move the constant to the right side and change its sign to be 9z= -6 -8. that’s the first step.the second step would be calculate the difference of -8-6 to get -14. you equation should now be 9z=-14the last step is to divide both sides by 9 to get z=-14/9hope this