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