Question 58099
Using: {{{log10(a)+log10(b) = log10(a*b)}}}

{{{log10(a*(a+21))=2}}}
{{{log10(a^2+21a))=2}}}
{{{10^2=(a^2+21a)}}}
{{{a^2+21a-100=0}}}
Using quadratic formula, taking the positive solution (see below) {{{a=4}}}
*[invoke quadratic x, 1, 21, -100]