Persamaan garis yang melalui (4,-1) dan (5,3) :

\begin{gathered} \frac{y - y1}{x - x1} = \frac{y2 - y1}{x2 - x1} \\ \frac{y + 1}{ x - 4} = \frac{3 + 1}{5 - 4} \\ \frac{y + 1}{x - 4} = \frac{4}{ 1} \\ y + 1 = 4x - 16 \\ y = 4x -17\end{gathered}x−x1y−y1=x2−x1y2−y1x−4y+1=5−43+1x−4y+1=14y+1=4x−16y=4x−17

m1 = 4

karena tegak lurus maka m2 = -1/m1 = -1/4

Persamaan garis yang melalui (2,1) :

y-y1 = m(x-x1)

y - 1 = -1/4(x -2)

y - 1 = -1/4x +1/2

1/4x + y -3/2 = 0 (kalikan 4)

x + 4y -6 = 0

4y + x = 6

(C)