Sometimes you have an algebraic expression with variables, and you know the values of those variables exactly, and you just need to plug in those values and get the value of the expression. Well, if that's what you have to do, then you've come to the right place, because this tutorial will show you exactly how to do it!
If 3x=21, what is 9x? To get the answer, solve the equation for the variable. Then plug that variable value into the expression and simplify to get the answer. Follow this same process anytime you need to use an equation to evaluate an expression. Watch this tutorial to see this process in action!
If you ever plug a value in for a variable into an expression or equation, you're using the Substitution Property of Equality. This property allows you to substitute quantities for each other into an expression as long as those quantities are equal. Watch this tutorial to learn about this useful property!