We can find the value of an expression when given the values of \(x\), \(y\), etc. To find the value of an expression, we need to substitute the given values for \(x\) and \(y\) into \(3x + 5y\).

A formula is a mathematical rule or relationship that uses letters to represent amounts which can be changed – these are called variables. For example, the formula to work out the area of a ...