You chose the coefficients of the variables x and y as your x-intercept and y-intercept. Instead, you need to substitute $\displaystyle{x}={0}\usepackage{cmbright}\Huge\$ into $\displaystyle{3}{x}-{5}{y}={15}\usepackage{cmbright}\Huge\$ to find the y-intercept, and substitute $\displaystyle{y}={0}\usepackage{cmbright}\Huge\$ to find the x-intercept.