from,
m = y - y1
x - x1
m = 2 - 5
4 - 1
m = -3 = -1
3
then,
y - y1 = m(x - x1)
using coordinates (1, 5)
y - 5 = -1(x - 1)
y = -x + 1 + 5
y = -x + 6.
