The equation of the line passing through point $ \left(1,~5\right) $, and point $ \left(2,~2\right) $ is:
$$ 3x+y-8=0 $$To find equation of the line passing through points $ A(x_A,y_A) $ and $ B(x_B,y_B) $, we use formula:
$$ y - y_A~=~\frac{y_B - y_A}{x_B - x_A}(x-x_A) $$In this example we have:
$$ \begin{aligned} & \left(1,~5\right) \implies x_A = 1 ~~\text{and}~~ y_A = 5 \\[1 em] & \left(2,~2\right) \implies x_B = 2 ~~\text{and}~~ y_B = 2 \end{aligned} $$After substituting into the formula, we obtain:
$$ \begin{aligned} y - y_A~&=~\frac{y_B - y_A}{x_B - x_A}(x - x_A) \\[1 em] y - 5~&=~\frac{ 2 - 5 }{ 2 - 1 } \left( x - 1 \right) \\[1 em]y - 5 ~&=~ -3 \left( x - 1 \right) \\[1 em]y - 5 ~&=~ -3x + 3 \\[1 em]y ~&=~ -3x + 8 \end{aligned} $$