amstex01

\documentclass{article}
\usepackage[fleqn]{amsmath} % dapat juga menggunakan leqno, reqno
\usepackage{mathrsfs,amsmath}
\begin{document}
\begin{eqnarray}
B(\alpha) &=& \frac{2}{\pi} \int_0^{\infty} f(z) \sin \alpha z \text{\, d}z\\
          &=& \frac{2}{\pi} \int_0^{\infty} 1. \sin \alpha z \text{\, d}z\\
          &=& \frac{2}{\pi} \int_0^{\infty} e^{-sz} \sin \alpha z \text{\, d}z ; \quad s=0\\
          &=& \frac{2}{\pi} \left[\mathscr{L}\{\sin \alpha z\}\right] ; \quad s=0\\
          &=& \frac{2}{\pi}. \frac{\alpha}{\alpha^2+s^2}; \quad s=0\\
          &=& \frac{2}{\pi}. \frac{\alpha}{\alpha^2}\\
          &=& \frac{2}{\pi \alpha}
\end{eqnarray}
\end{document}