The y-intercept is 20, as the line is on the y-axis while x is 0. The slope would be 0, because the y and x values both increase by 10, so 10/10 which simplifies to 1. The result would be y=10/10x + 20 simplifying to y=1x+20, which is y=x+20.
Per hour, he snores 3 times per hour since he snores once every 20 minutes. In 3 hours, there are 180 minutes, so 180/20 = 9. The dog snores 9 times in 3 hours.