Answer
$L(d)=\frac{5}{7}d$
Work Step by Step
There is an equal proportion of the heights and their bases: $\frac{L+d}{L}=\frac{12}{5}$, so one just needs to solve for L to find the equation
$\frac{L}{L}+\frac{d}{L}=\frac{12}{5}$
$1+\frac{d}{L}=\frac{12}{5}$
$1-1+\frac{d}{L}=\frac{12}{5}-1$
$\frac{d}{L}=\frac{7}{5}$
$\frac{d}{L}\cdot L \cdot\frac{5}{7} =\frac{7}{5}\cdot \frac{5}{7}\cdot L$
$d \cdot\frac{5}{7} =L$
$L(d)=\frac{5}{7} d$