Correct. Let $\displaystyle{m}=\usepackage{cmbright}\Huge\$ the number of math books and let $\displaystyle{h}=\usepackage{cmbright}\Huge\$ the number of history books. Then:

(A) $\displaystyle{m}+{h}={30}\usepackage{cmbright}\Huge\$ (B) $\displaystyle{3}{m}+{2}{h}={77}\usepackage{cmbright}\Huge\$

Solve (A) for h: $\displaystyle{h}={30}-{m}\usepackage{cmbright}\Huge\$

Substitute into (B): $\displaystyle{3}{m}+{2}{\left({30}-{m}\right)}={77}\usepackage{cmbright}\Huge\$

Multiply, and combine like terms: $\displaystyle{m}+{60}={77}\usepackage{cmbright}\Huge\$ , so $\displaystyle{m}={17}\usepackage{cmbright}\Huge\$