Using FOIL: $\displaystyle{\left({3}{x}+{7}\right)}{\left({x}+{4}\right)}={3}{{x}}^{{2}}+{12}{x}+{7}{x}+{28}={3}{{x}}^{{2}}+{19}{x}+{28}\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.