One die has a 5. The only way to get a sum of 7 is by rolling a 2 on the other die. The other die has 6 equally likely outcomes, the numbers 1, 2, 3, 4, 5, or 6. The probability of rolling a 2 with one die is 1/6.
Get the slope from the equation by getting y by its self then dividing everything by -1, which gives you y=-3x-12. You the -3 as your slope when doing the point-slope form.