3x+5x ? 8y+3y ? 2n-3n ? 7z-10z ?
There is a concept called the degree of a variable. the variable is the letter (x,y,n,z,etc.) you can only add/subtract/multiply/divide common variables.
x's to x's etc.
if there is a power that counts too.
x^2 to x^2.
I hope this helps