An equation can be used to find the height of a child. Equation is h=2c. The child is 36 inches at 18 months. (12in=1ft) How tall will the child be when he is 5 years old?

If this relation were to be even remotely true, we could form a ratio ....

18 months/36 inches = 60 months/x inches
18x = 2160
x = 120 inches or 10 ft tall

(can you see how absurd your question is ???? )

Also in
h = 2c , you don't define what h and c stand for.