Answer
$d=\frac{l}{f}$
Work Step by Step
Multiply $d$ to both sides of the equation:
$d \cdot f=d \cdot \frac{l}{d}
\\df = l$
Divide $f$ to both sides of the equation:
$\frac{df}{f}=\frac{l}{f}
\\d=\frac{l}{f}$
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