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log18 = A
log(2 x 3²) = A
log2 + 2 log3 = A ...... [1]

log21 = B
log(3 x 7) = B
log3 + log7 = B ...... [2]

log28 = C
log(2² x 7) = C
2log2 + log7 = C ...... [3]

[3] - [2] :
2 log2 - log3 = - B + C
4 log2 - 2 log3 = -2B + 2C ...... [4]

[1] + [4] :
5 log2 = A - 2B + 2C
log2 = (A - 2B + 2C)/5
2 log2 = (2A - 4B + 4C)/5
log(2^2) = (2A - 4B + 4C)/5
log4 = (2A - 4B + 4C)/5

log84
= log(21 x 4)
= log21 + log4
= B + [(2A - 4B + 4C)/5]
= (5B/5) + [(2A - 4B + 4C)/5]
= (2A + B + 4C)/5