



 
Hi Jeff, I drew a diagram of the face of your wall. The height at the point $P$ is 24 inches and the height at point $R$ is 116 inches. $Q$ is midway from $P$ to $Q.$ Since the top of the wall is a straight line from $a$ to $b$ the average height is the height at $Q$ which is the average of the height at $P$ and the height at $R.$ Hence the average height is \[\frac{24 + 116}{2} = 70 \mbox{ inches.}\] As a justification I drew a red horizontal line 70 inches above the base. Notice that the two green triangles are identical (congruent) I hope this helps, 



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