You may have ignored the negative signs when substituting the point into the inequality. Substituting $\displaystyle{\left(-{3},-{4}\right)}\usepackage{cmbright}\Huge\$ into $\displaystyle{2}{y}-{5}{x}<{3}\usepackage{cmbright}\Huge\$ gives you $\displaystyle-{8}+{15}<{3}\usepackage{cmbright}\Huge\$ , which is a false statement. So $\displaystyle{\left(-{3},-{4}\right)}\usepackage{cmbright}\Huge\$ is not a solution to the inequality.