In this module you can study how to substitute numbers into algebraic expressions.
The above video from Khan Academy explains how to evaluate expressions with one variable.
The above video from Khan Academy explains how to evaluate expressions with two variables.
An expression is not an equation.
You cannot solve an expression; you can only simplify it or evaluate it.
Some people try adding "= 0 " to the end of an expression when they want to "solve" it, but you cannot solve an expression.
Use these search terms to find additional resources:
evaluating expressions
algebraic expressions