On a coordinate plane, what is the difference between (-2,7) and (-2,-6)?

If you mean distance, then we can use the distance formula to find out.

(x - x)² + (y - y)² = d²   Distance formula
(-2 + 2)² + (7 + 6)² = d²   Substitute the points; remember that when you subtract a negative, you add, and also remember to always subtract in the same direction (with the points (1,2) and (3,4), if you do 1 - 3, then do 2 - 4 and not 4 - 2)
(0)² + (13)² = d²   Add
0 + 169 = d²   Square
169 = d²   Add
13 = d   Take the square root of both sides to cancel out the exponent

The distance between (-2,7) and (-2,-6) is 13 units.

Hope this helps!