Question 379855
Just treat b as it were any other constant, such as 10 or 26.3.

We have
{{{2e^(3x) + e^(3x) = b}}}
{{{3e^(3x) = b}}}
{{{e^(3x) = b/3}}}
{{{3x = ln (b/3)}}}
{{{x = (1/3)(ln(b/3)) = (1/3)(ln b - ln 3)}}}