Like terms in algebra are terms that have the same variables and power. Constants contain only numbers, we call them constants because their values don't change. Coefficients are the numbers in front of the variable ex: 5y in that expression 5 is the coefficient. So in the expression 2+3a+9a there is one constant which is 2 the likes terms are 3a and 9a because they have the same variable. And the coefficients are 3 and 9 because they are in front of the variable.