TeX source:

 \begin{aligned} &S V_{T}=E S_{T}-A T \\ &S P I_{T}=\frac{E S_{T}}{A T}=\frac{E S_{T}}{E S_{T}-S V_{T}}=\frac{W_{T}^{*} B P D_{0}}{W_{T}^{* B P D_{0}-P_{T}^{*} S B}} \end{aligned}