this is so confusing i do not understand what this even is

Since the triangle is dilated, the sides of the original triangle are stretched. Since J'K' is the length of one side of the triangle after the dilation, we have that:
[tex]\begin{gathered} J^{\prime}K^{\prime}=\frac{4}{3}JK,J^{\prime}K^{\prime}=13.5 \\ \Rightarrow13.5=\frac{4}{3}JK \\ \Rightarrow JK=\frac{3\cdot13.5}{4}=10.125 \end{gathered}[/tex]Then, the answer is 10.125