Question

u-1/u-4 - 4/u^2-16?

Answer 1

Must use brackets.
if you mean:
\[\frac{u-1}{u-4}-\frac{4}{u^2-16}\]
then you can rewrite it:
\[\frac{u-1}{u-4}-\frac{4}{(u-4)(u+4)}=\frac{(u-1)(u+4)}{(u-4)(u+4)}-\frac{4}{(u-4)(u+4)}=\frac{u^2-u+4u-4-4}{(u-4)(u+4)}\]

Answer 2

\[\frac{u^2+3u-8}{u^2-16}\]