[tex]\text{Find the one bottom side of the triangle first}\\\\\text{We know that the total length is 21, but part of it is 11, so subtract}\\\\21-11=10\\\\\text{Divide 10 by 2 because there are two sides for two triangles}\\\\10\div2=5\\\\\text{Now, since we know two of the sides, we would plug in the values}\\\text{into the Pythagorean theorem and solve:}\\\\5^2+b^2=13^2\\\\25+b^2=169\\\\\text{Subtract 25 from both sides}\\\\b^2=144\\\\[/tex]
[tex]\text{Square root both sides}\\\\\sqrt{b^2}=\sqrt{144}\\\\b=12\\\\\text{This means that x = 12}\\\\\boxed{x=12}[/tex]
