To add the probabilities, you would have to treat them like any other numbers. If they are decimals or percents, you can follow basic addition rules. If you are given the probabilities as fractions, make sure that you have common denominators when you add them.
Answer:
The slope is just 2/3 and the y intercept is (0,3)
4(3+5) --> this means that 12/4=3 and 20/4=5 and 4 is the GCF