'When 8 is added to a number m, the result is 21.' Which equation says this?

To evaluate an expression, **substitute** a value for the variable and compute. If $x=5$, then $2x+3$ becomes $2\times 5+3=10+3=13$. The same expression with $x=0$ gives $2\times 0+3=3$. Practise substituting — it is the bridge between an algebraic expression and a real number.