h(20) = 68
h'(20) = -3
how to find h(22)?
Well, you can only guess by assuming a straight line of form
h = m x + b
here m = -3
so
h = -3 x + b
put in your point (20,68)
68 = -3(20) + b
68 = -60 + b
b = +128
so
h = -3 x +128
h = -66 + 128 = 62
alternatively you can use common sense.
h is at 68 and h goes down 3 for every 1 increase in x.
so when x goes up 2, h goes down 6
so
68 - 6 = 62
when it goes up