To determine the corresponding expression to 7(1+3) distribute the multiplication on the parentheses term. This means that you have to multiply each term inside the parentheses (1 and 3) by 7:
[tex]\begin{gathered} 7(1+3) \\ 7\cdot1+7\cdot3 \\ 7+21 \end{gathered}[/tex]
The corresponding expression is 7 + 21
