Question 22906
Recall that:

{{{log(b,M)=x}}} means: {{{b^x = M}}} Applying this to your problem:

{{{log(10,(3x-7)) = 21}}} means:
{{{10^21 = (3x-7)}}} Add 7 to both sides.
{{{10^21 + 7 = 3x}}} Finally, divide both sides by 3.
{{{x = (10^21 + 7)/3}}}