答拉暑作唔該曬啦

(2^x)(3^x) = 2100
log [(2^x)(3^x)] = log 2100
log (2^x) + log(3^x) = log 2100
x(log 2) + x(log 3) = log (21*100)
x(log 2 + log 3) = log 21 + log 100
x = (2 + log 21)/(log 2 + log 3)

Hope this helps =]

2012-07-18 15:21:22 補充：
x = (2 + log 21) / (log 2 + log 3)
x = (2 + log(3*7)) / (log 2 + log 3)
x = (2 + log 3 + log 7) / (log 2 + log 3)

2012-07-18 15:23:12 補充：
all logarithms above have base of 10

2100 = 2² × 3 × 5² × 7