If you use the slope formula,
y - y1 = m ( x - X1) and substitute your points in (0,5)
The 5 is substituted in for y1 and the 0 is substituted in for X1.
So you get y (-5) = (-1) (x - 0)
If you add 5 to both sides, you get
the equation.
I hope that's right lol