Answer:
log₁₀(M) + log₁₀(N)
Explanation:
The rules of logarithms tell you ...
log(ab) = log(a) +log(b) . . . . . base of logarithms the same in each case
When your log is log₁₀ and a=M and b=N, this becomes ...
log₁₀(MN) = log₁₀(M) + log₁₀(N)
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Some functions are not always written with their arguments in parentheses. I believe it avoids ambiguity to put function arguments in parentheses, so I have done that here. It appears your answer form may want you to write ...
log₁₀M+log₁₀N . . . . . I don't recommend this form