The equation of the line passing through point $ \left(12,~12\right) $, and point $ \left(0,~12\right) $ is:
$$ y-12=0 $$In this example we have:
$$ \begin{aligned} & \left(12,~12\right) \implies x_A = 12 ~~\text{and}~~ \color{blue}{y_A = 12 } \\[1 em] & \left(0,~12\right) \implies x_B = 0 ~~\text{and}~~ \color{blue}{y_B = 12 } \end{aligned} $$Since, $ y_A = y_B $ the equation of the line is:
$$ y = y_A $$In this example this is:
$$ y = 12 $$