To follow up, the \(n\)th term may be written as:
[MATH]2^{12-n}k^{-n}{12 \choose n}x^{48-2n}[/MATH]
The term in \(x^{40}\) must have \(n=4\), and the term in \(x^{38}\) must have \(n=5\), and the ratio of the coefficient of the former term to the latter term is 5:
[MATH]\frac{2^{12-4}k^{-4}{12 \choose 4}}{2^{12-5}k^{-5}{12 \choose 5}}=5[/MATH]
[MATH]\frac{2^{8}k^{-4}\dfrac{12!}{4!8!}}{2^{7}k^{-5}\dfrac{12!}{5!7!}}=5[/MATH]
[MATH]\frac{2k\cdot5}{8}=5[/MATH]
[MATH]\frac{k}{4}=1[/MATH]
[MATH]k=4[/MATH]