I am building a 12x20 saltbox shed. The front wall is 14.25" higher than the rear wall. I would like to have my front RUN 5' and he back run 7' and I am having a hard time trying to figure out the height of my ridge board for a 6/12 pitch from the top plate to bottom of 2x8 ridge with 2x6 rafters. Thanks.

The 5 and 7 ft. are hypotenuse so they would be longer then that. A 6/12 would increase that even more. Lay it out on graph paper. 12 " across. Then run a parallel line at 14.25" above the 12 ft. base. Mark your 5 ft. along the base and extend above the parallel line until you get a 6/12 pitch. Here is a link

https://www.mycarpentry.com/roof-pitch-calculator.html
Just as easy is to use a degree system, 6/12 is 26.5*. Lay out the base line, set a perpendicular line at 5' and use a protractor set to 26.5* and see where the angle line contacts the perpendicular line. Now you have 3 sides and can also measure the roof rafter.