In algebra, variable is a letter, like x,y,z but x is the most common variable that teachers used in algebraic expressions.
To evaluate algebra expression you have to substitute the variable into the number and try the arithmetic method.
Like for example:
But if the teacher is already show the values of the variables, we just simply substitute it into a number...
Calculate the expression for y=2 and z=3
3y + 5z= ?
3(2) + 5(3)=? Substitute the y to 2 and z to 3