Add 15 to all parts: $\displaystyle-{5}+{15}<-{5}{k}-{15}+{15}<{20}+{15}\usepackage{cmbright}\Huge\$

Simplify to: $\displaystyle{10}<-{5}{k}<{35}\usepackage{cmbright}\Huge\$

Divide all parts by $\displaystyle-{5}\usepackage{cmbright}\Huge\$ , and reverse the inequality signs.

$\displaystyle\frac{{{10}}}{{-{{5}}}}>\frac{{-{5}{k}}}{{-{{5}}}}>\frac{{{35}}}{{-{5}}}\usepackage{cmbright}\Huge\$

so $\displaystyle-{2}>{k}>-{7}\usepackage{cmbright}\Huge\$

or $\displaystyle-{7}<{k}<-{2}\usepackage{cmbright}\Huge\$