The slope m of a straight line passing through points (x₁,x₂) and (y₁,y₂) can be calculated like this:
m = change in y-value / change in x-value
m = (y₂ - y₁) / (x₂ - x₁)
We are given the points as (7,r) and (4,6) and slope m=0 . Substituting these:
⇒m = 6 - r / 4 - 7
⇒0 = 6 - r / -3
∴ r = 6