Answer: 20
H(c) = 6.4 + 0.6c
6.4 is the constant.
When the height of the cups is 18.4 the function is:
18.4 = 6.4 + 0.6c
Then, you add 6.4 from both sides
18.4 - 6.4 + 6.4 = 6.4 + 0.6c - 6.4 + 6.4
Simplify
18.4 = 6.4 + 0.6c
Switch sides
6.4 + 0.6c = 18.4
Multiply both sides by 10
6.4 x 10 + 0.6c x 10 = 18.4 x 10
Refine
64 + 6c = 184
Subtract 64 from both sides
64 + 6c - 64 = 184 - 64
Simplify
6c = 120
Divide both sides by 6
6c/6 = 120/6
c = 20