Add up the other angle or angles. take 180 and subtract from what you got
Seems like you already have the answer so peace
The keyword is "sum" which tells you x, y, and z are being added. One half of the sum means that the sum of the three numbers is going to be divided by 2.
x + y + z
------------
2
Let the width of the frame = x
Let the cost of a linear foot of the frame = y
Let the inside dimension of the length of the frame = a
Let the inside dimension of the width of the frame = b.
The total length of the frame = 2*a + 4x
The total width of the frame = 2b
The total amount of the frame in feet is (2a + 4x + 2b)*y = Cost
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