Here are two ways (of very many):
Column[
{Underscript[
Graphics[{EdgeForm[Thick], White, Triangle@{{0, 0}, {3, 8}, {10, 0}}}, ImageSize -> 400],
Style[\[UnderBrace], 100, FontFamily -> "Vladimir Script"]],
Pane[Style["comment about triangle", Black, Bold, 20], 385, Alignment -> .2]}]
Graphics[
{EdgeForm[Thick], White, Triangle@{{0, 0}, {3, 8}, {10, 0}},
Text[Overscript[Pane[Style["comment about triangle", Black, Bold, 20], 385, Alignment -> .2],
Style[\[UnderBrace], Black, 100, FontFamily -> "Vladimir Script"]], {5, -.9}]},
ImageSize -> 400]