log2(3)=a ---->log3(2)=1/a
log3(7)=b
log12(56)=log3(56)/log3(12)
=log3(7*2³)/log3(3*2²）
=【log3(7)+3log3 (2)】/[log3(3)+2log3(2)]
=(b+3/a)/(1+2/a)
=(ab+3)/(a+2)