Resposta :
a² = b² + c²
13² = 5² + x²
169 = 25 + x²
169 - 25 = x²
144 = x²
x = √144
x = 12cm
[tex]\large\boxed{\begin{array}{l} \rm \: x {}^{2} = 13 {}^{2} - 5 {}^{2} \\ \rm \: x {}^{2} = 169 - 25 \\ \rm \: x {}^{2} = 144 \\ \rm \: x = \sqrt{144} \\ \rm \: x = 12 \: cm \end{array}}[/tex]