The difference in the x-coordinates of two points is 3, and the difference in the y-coordinates of the two points is 6. what is the slope of the line that passes through the points?

They're giving you the answer right in the question!
Think about the slope formula: y2-y1/x2-x1. Surely you know that the answer to a subtraction question is referred to as the difference! Since, they said the difference between the two x-coordinates is 3 and difference between the two y-coordinates is 6, you would just plug those right into the slope formula! 6/3 equals 2. 
The slope of the line that passes through the points would be 2!

The answer should be c.