What is the equation of a line that crosses the Y axis at zero -3 and has a slope of 4

How do you answer this question? (step by step)

7. if the value of x varies directly with the value of y, and x = 3 when y = 21. what is the valu y, and x = 3 when y = 21. what is the value of x when y = 105?

Which expression gives you the distance between the points (5,1)and(9,-6)