Step 1
Longest side is the hypotanuse = x
opposite = 5
Adjacent = 12
Apply pythagorus theorem
[tex]\begin{gathered} \text{Opposite}^2+Adjacent^2=Hypotanuse^2 \\ 5^2+12^2=x^2 \\ \\ 25+144=x^2 \\ x^2\text{ = 169} \\ x\text{ = }\sqrt[\square]{169} \\ x\text{ = 13} \end{gathered}[/tex][tex]undefined[/tex]