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If the equation you're looking for is y = m * ln(x) + b and your x and y inputs are in A2:A100 and B2:B100 respectively, you can find m and b by using INDEX and LINEST together:
m =INDEX(LINEST(B2:B100,LN(A2:A100)),1)
b =INDEX(LINEST(B2:B100,LN(A2:A100)),1,2)