Answer:
a) 39a - 7 = 9a + 5a+1 + 17a-6 + X
b) 8a - 2
Explanation:
Let X be the length of the fourth side
39a - 7 = 9a + 5a+1 + 17a-6 + X
39a - 7 = 31a - 5 + X
X = 39a - 7 - 31a + 5
X = 8a - 2
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