Rolling a number less than 4 means that rolling a 1, 2, or 3 will satisfy the requirement. Since 3 of 6 possible outcomes will satisfy the requirement then the likelihood that it will be rolled is 3/6 times. 3/6 reduces to 1/2 or 50% chance.
I can't directly tell you the answer, but I can help you for how to do it.
So every single time you want to find the rate of change (slope) label 2 points with x1, y1, x2 and y2. We can use the point-slope form to sind the slope:
y1-y2/x1-x2
And we just need to plug in the point to get the Y intercept.
When the x is 0, the y value will be the y-intercept.
The slope intercept form is: y=Mx+B. M stands for slope and B stand for y-intercept. Just plug the slope and y-intercept in and get the answer.