AMC 12A 2010 Problem 11

Past Contests AMC 12 AMC 12A - 2010
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1

The solution of the equation 7^{x+7}=8^{x} can be expressed in the form x=\log _{b} 7^{7}. What is b ?

Answer Choices
A. \dfrac{7}{15}
B. \dfrac{7}{8}
C. \dfrac{8}{7}
D. \dfrac{15}{8}
E. \dfrac{15}{7}