Let's go:
y = ax + b
{5 = 4a + b
{15 = - 6a + b
{- 15 = 6a - b
10a = - 10
a = - 1
5 = 4a + b
4.(-1) + b = 5
b - 4 = 5
b = 9
- 1; 9
y = -1x + 9
I hope I helped you.
