The difference between an equation and expression is that an equation has an equals sign.
To simplify an expression like terms are collected together.
Simplify 2x^2 + 6y - x^2 + 4y - 6
Collect the like terms; 2x^2 - x^2 + 6y + 4y - 6
The x terms can be simplified: 2x^2 - x^2 = x^2