Height of the trans American pyramid = a
Height of the space needle = s
Height of the gateway arch = g
Given,
s + 76 = a
and,
s + 8 = g
Also,
s = a - 76
s = g - 8
Now,
a - 76 = g - 8
a - g = -8 + 76
a - g = 68
(s + 76) - g = 68
(s + 76) - g = 68
s + 76 - g = 68
s - g = 68 - 76
s - g = -8
(g - 8) - g = -8