how to align equal signs?

Try,

\begin{array}{rcl}
 a^2+b^2&=&c^2\\
19^2+b^2&=&27^2\\
27^2-19^2&=&b^2\\
720-361&=&b^2\\
\sqrt{368}&=&\sqrt{b}\\
b&=&19.183
\end{array}

You can change the {rcl} in the first line to get alternative alignments. 'r'=right; 'c'=center, 'l'=left If you use {r|c|l} the you get vertical lines between columns. Equally you can add horizontal lines between rows with \hline

Hope this helps