The slope-intercept form of the equation of a line is
y = mx + b
where m = slope, and b = y-intercept.
We are given the slope, m = 4, so we can already substitute 4 for m in the equation above giving us
y = 4x + b
Now we need to find b.
Since we are given a point on the line, (6, -1), we substitute x and y with the x- and y-coordinates of the point, respectively, and solve for b.
From our point, we have x = 6, and y = -1.
y = 4x + b
-1 = 4(6) + b
-1 = 24 + b
-25 = b
b = -25
Now that we know that b = -25, we substitute b with -25 in y = 4x + b to get our answer:
y = 4x - 25