Solve (d-1)/5 = (d-3)/3

$\frac{d-1}{5}=\frac{d-3}{3}$

Cross-multiply.

$(3)(d-1)=(5)(d-3)$

Distribute.

$3d-3=5d-15$

Subtract 3d from both sides.

$-3=5d-3d-15$

Add 15 to both sides.

$-3+15=5d-3d \\~\\ 12=2d$

Divide both sides by 2.

$\frac{12}{2}=d \\~\\ 6=d$