You probably found the least common denominator $\displaystyle{\left({x}+{4}\right)}{\left({x}-{1}\right)}\usepackage{cmbright}\Huge\$ and attempted to rewrite each fraction with this denominator. However, you forgot to apply the distributive property in each numerator:

$\displaystyle\frac{{{3}{x}}}{{{x}+{4}}}+\frac{{1}}{{{x}-{1}}}=\frac{{{\left({3}{x}\right)}{\left({x}-{1}\right)}}}{{{\left({x}+{4}\right)}{\left({x}-{1}\right)}}}+\frac{{{1}{\left({x}+{4}\right)}}}{{{\left({x}+{4}\right)}{\left({x}-{1}\right)}}}\usepackage{cmbright}\Huge\$

Don't leave out $\displaystyle{\left({3}{x}\right)}{\left(-{1}\right)}\usepackage{cmbright}\Huge\$ .