Using FOIL: $\displaystyle{\left({7}{x}+{3}\right)}{\left({x}+{4}\right)}={7}{{x}}^{{2}}+{28}{x}+{3}{x}+{12}={7}{{x}}^{{2}}+{31}{x}+{12}\usepackage{cmbright}\Huge\$

This is not equal to the given expression. First group terms into pairs and factor out the GCF of each pair. Then factor out the common binomial factor.