Question 309606
Start with:
{{{A = (1/2)b*h}}} Substitute {{{A = 24}}} and {{{h = 3b}}} because if {{{b = (1/3)*h}}} then {{{h = 3b}}}
{{{24 = (1/2)(b)(3b)}}} 
{{{24 = 3b^2/2}}} Multiply both sides by 2.
{{{48 = 3b^2}}} Divide both sides by 3.
{{{16 = b^2}}} Take the square root of both sides, discarding the negative solution.
{{{b = 4}}}
The base is 4 meters long.