The equation of the line passing through point $ \left(2,~9\right) $, and point $ \left(4,~8\right) $ is:
$$ x+2y-20=0 $$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:
$$ \begin{aligned} & \left(2,~9\right) \implies x_A = 2 ~~\text{and}~~ y_A = 9 \\[1 em] & \left(4,~8\right) \implies x_B = 4 ~~\text{and}~~ y_B = 8 \end{aligned} $$After substituting into the formula, we obtain:
$$ \begin{aligned} y - y_A~&=~\frac{y_B - y_A}{x_B - x_A}(x - x_A) \\[1 em] y - 9~&=~\frac{ 8 - 9 }{ 4 - 2 } \left( x - 2 \right) \\[1 em]y - 9 ~&=~ -\frac{ 1 }{ 2 } \left( x - 2 \right) \\[1 em]y - 9 ~&=~ -\frac{ 1 }{ 2 }x + 1 \\[1 em]y ~&=~ -\frac{ 1 }{ 2 }x + 10 \end{aligned} $$