The height H of a child as a function of the circumference of the head is given by H(C) = 2.15C – 10.53

Let y = H(C) = 2.15C – 10.53

=> 2.15C = y + 10.53

=> C = (y + 10.53)/2.15

From this we get C(H) = (H + 10.53)/2.15

H(C(H)) = H((H + 10.53)/2.15) = 2.15*((H + 10.53)/2.15) – 10.53

=> H + 10.53 - 10.53 = H

C(H(C)) = C(2.15C – 10.53) = (2.15C – 10.53 + 10.53)/2.15 = C

If the height of the child is 26 inches, the circumference of the head is C(26) = (26 + 10.53)/2.15 = 16.99 inches approximately.

The circumference of the head as a function of height is C(H) = (H + 10.53)/2.15 and the circumference for a height 26 inches is 16.99 inches approx.