\[3(7\text{ }+~i7)\text{ }+~i(7\text{ }+~i7)~\]
\[=\text{ }21\text{ }+~i21\text{ }+~i7\text{ }+~{{i}^{2~}}7\]
\[=\text{ }21\text{ }+~i28\text{ }\text{ }7~\]
\[[{{i}^{2}}~=\text{ }-1]\]
\[=\text{ }14\text{ }+~i28\]
Consequently,
\[3\left( 7\text{ }+\text{ }i7 \right)\text{ }+\text{ }i\left( 7\text{ }+\text{ }i7 \right)\text{ }=\text{ }14\text{ }+\text{ }i28\]