Answer
$M=7$
Work Step by Step
RECALL:
The change-of-base formula:
$\log_a{b} = \dfrac{\log_c{b}}{\log_c{a}}$
Using the formula above gives:
$\log_8{7}=\dfrac{\log_5{7}}{\log_5{8}}$
Thus,
$M=7$.
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