You have the following expression;
[tex](2+4i)(2+4i)[/tex]to perform the previous operation, expand the previous factors as follow:
[tex]\begin{gathered} (2+4i)(2+4i)=(2)(2)+(2)(4i)+(4i)(2)+(4i)(4i) \\ =4+8i+8i+16i^2 \\ =4+16i+16i^2 \end{gathered}[/tex]take into account that i^2 = -1, then, you obtain:
[tex]4+16i+16(-1)=4+16i-16=16i-12[/tex]Hence, the result is 16i - 12
