They're just dividing each side of [MATH]A\sin(pL) = \frac{V}{P}L[/MATH] by each side of [MATH]Ap\cos(pL) = \frac{V}{P}[/MATH], to get [MATH]\frac{A\sin(pL)}{Ap\cos(pL)} = \frac{\frac{V}{P}L}{\frac{V}{P}}[/MATH].
The LHS becomes [MATH]\frac{A\sin(pL)}{Ap\cos(pL)} = \frac{A}{Ap}\frac{\sin(pL)}{\cos(pL)} = \frac{1}{p}\tan(pL)[/MATH].
The RHS becomes [MATH]\frac{V}{P}L\div\frac{V}{P} = \frac{V}{P}L\cdot\frac{P}{V} = L[/MATH].
So they end up with [MATH]\frac{1}{p}\tan(pL) = L[/MATH], which is equivalent to [MATH]\tan(pL) = pL[/MATH].