Answer: choice C) 15
Simplify the left side to get
2(4+x)+(13+x)
2(4)+2(x) +13+x
8+2x+13+x
3x+21
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So the original equation
2(4+x)+(13+x) = 3x+k
turns into
3x+21 = 3x+k
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Subtract 3x from both sides
3x+21 = 3x+k
3x+21-3x = 3x+k-3x
21 = k
k = 21
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If k = 21, then the original equation will have infinitely many solutions. This is because we will end up with 3x+21 on both sides, leading to 0 = 0 after getting everything to one side. This is a true equation no matter what x happens to be.
If k is some fixed number other than 21, then there will be no solutions. This equation is inconsistent (one side says one thing, the other side says something different). If k = 15, then
3x+21 = 3x+k
3x+21 = 3x+15
21 = 15 .... subtract 3x from both sides
The last equation is false, so there are no solutions here.
note: if you replace k with a variable term, then there will be exactly one solution.