Straight line:
y = mx + c
m is the slope of the graph and c is the y-intercept
In this case, m = 5 as stated in the question, so...
y = 5x + c
By substituting the given co-ordinates (-2,-1) into this equation, we can find c
-1 = 5(-2) + c
-1 = -10 + c
-1 + 10 = -10 + c + 10 (Add 10 to both sides)
9 = c
c = 9
Put c = 9 into the equation:
y = 5x + 9