Usemos el teorema de Pitágoras y nos queda :
<img src="https://tex.z-dn.net/?f=A%3Db%5C%5CB%3D%5Cfrac%7B3%7D%7B4%7Db%5C%5CC%3D10%5C%2Cm%5C%5CC%5E%7B2%7D%3DA%5E%7B2%7D%2BB%5E%7B2%7D%5C%5C%2810%29%5E%7B2%7D%3D%28b%29%5E%7B2%7D%2B%28%5Cfrac%7B3%7D%7B4%7Db%29%5E%7B2%7D%5C%5C100%3Db%5E%7B2%7D%2B%5Cfrac%7B9%7D%7B16%7Db%5E%7B2%7D%5C%5C100%3D%5Cfrac%7B25%7D%7B16%7Db%5E%7B2%7D%5C%5Cb%5E%7B2%7D%3D%28100%29%28%5Cfrac%7B16%7D%7B25%7D%29" />
Despejando b nos queda :
<img src="https://tex.z-dn.net/?f=b%3D%5Csqrt%7B%28100%29%28%5Cfrac%7B16%7D%7B25%7D%29%7D%5C%5Cb%3D%5Csqrt%7B64%7D%5C%5Cb%3D8%5C%2Cm" />
Así las dimensiones son :
<img src="https://tex.z-dn.net/?f=A%3D8%5C%2Cm%5C%5CB%3D%5Cfrac%7B3%7D%7B4%7D%288%5C%2Cm%29%3D6%5C%2Cm" />
Comprobando :
<img src="https://tex.z-dn.net/?f=A%5E%7B2%7D%2BB%5E%7B2%7D%3DC%5E%7B2%7D%5C%5CC%3D%5Csqrt%7BA%5E%7B2%7D%2BB%5E%7B2%7D%7D%5C%5CC%3D%5Csqrt%7B8%5E%7B2%7D%2B6%5E%7B2%7D%7D%3D%5C%5CC%3D%5Csqrt%7B64%2B36%7D%3D%5C%5CC%3D10%5C%2Cm" />
Saludos.