In this example, it's not an equation (it doesn't "equal" anything), just an expression. We cannot combine z with the actual numbers. It is a variable, so it could be combined with other terms including z, like if it said 2z + z + 5 x 3.
The only thing you can do with your expression is "combine like terms" by multiplying 5 and 3 to get 15.
I hope this helped. Happy to answer questions you may have.
