The y-intercept represents the initial girth of the tree. Assuming that the X-axis represents time in this scenario (as it usually does) the y-axis, therefore, represents the girth of the tree. The y-intercept is located where X = 0. So if no time has passed then this must be the first measurement of the girth of the tree.
It takes 3600 seconds for one hour. It takes 14400 seconds for four hours.
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The distributive property:
a(b + c) = ab + ac
2(x - 8) = 68
(2)(x) - (2)(8) = 68
2x - 16 = 68 |+16
2x = 84 |:2
x = 42
Answer: D. 42
