So remember the formula for working out the slope:
(y2 - y1) ÷ (x2 - x1)
We already know two points:
(11, 5) and (20, 5) (Remember points are like (x, y)
Therefore y2 = 5 and y1 = 5 and x2 = 20 and x1 = 11
Substitute these into the formula from the start:
(y2 - y1) ÷ (x2 - x1)
(5 - 5) ÷ (20 - 11)
0 ÷ 9
And we can determine the slope is equal to 0 as 0 ÷ 9 = 0