In this case x is the geometric mean & as per the formula geometric mean of 2 numbers =√(a₆ * a₈) = √(12*27) & x=18. Proof: Find the common ratio between a₇ & a₆ ==> = ( x) / 12 now find the common ration between a₈ & a₇ ===> =27 / (x) x/12 = 27/x ===> x² = 12*27 ==> x² = 324 & x =√324 = 18
