Explanation:
the y-intercept (b) is the easiest :
it is the y-value, when x = 0 : -3
the slope (m) is the ratio (y coordinate difference / x coordinate difference) when going from 1 point to another.
so, we need to identify 2 points in the line.
one we have already with b : (0, -3)
another is the x-axis intercept, which is clearly (4, 0)
the difference is not important, but we need to use the same for both (x and y).
I prefer to go from left to right, so from (0, -3) to (4, 0) :
x changes by +4 (from 0 to 4)
y changes by +3 (from -3 to 0)
the slope is +3/+4 = 3/4