They receive x and need 3 more, so $\displaystyle{x}+{3}\usepackage{cmbright}\Huge\$ is the number they needed before the order. They originally needed 25 and had 7 already, so $\displaystyle{25}-{7}\usepackage{cmbright}\Huge\$ is also the number they needed before the order. Since $\displaystyle{x}+{3}\usepackage{cmbright}\Huge\$ and $\displaystyle{25}-{7}\usepackage{cmbright}\Huge\$ represent the same number, they are equal to each other.