TeX source:

\begin{gathered} \sum_{j=3}^{5}(5+f(j-2))=(5+f(3-2))+(5+f(4-2))+(5+f(5-2))=(5+f(1))+(5+f(2))+(5+f(3)) \\ =(5+2)+(5+3)+(5+1)=21 \end{gathered}