Como se resuelve esto?

Recordemos la expresión que relaciona el área de un trapecio :

<img src="https://tex.z-dn.net/?f=A%3D%5Cfrac%7B%28B%2Bb%29h%7D%7B2%7D" />

Luego :

<img src="https://tex.z-dn.net/?f=A%3D49%5C%2Ccm%5E%7B2%7D%5C%5CB%3D5%5C%2Ccm%5C%5Cb%3D5x-3%5C%2Ccm%5C%5Ch%3D14%5C%2Ccm" />

Despejando b se tiene :

<img src="https://tex.z-dn.net/?f=2A%3D%28B%2Bb%29h%5C%5C%5Cfrac%7B2A%7D%7Bh%7D%3DB%2Bb%5C%5C%5Cfrac%7B2A%7D%7Bh%7D-B%3Db" />

Sustituyendo los valores previos se obtiene :

<img src="https://tex.z-dn.net/?f=5x-3%3D%5Cfrac%7B2%2849%5C%2Ccm%5E%7B2%7D%29%7D%7B14%5C%2Ccm%7D-5%5C%2Ccm%5C%5C5x%3D%5Cfrac%7B2%2849%5C%2Ccm%5E%7B2%7D%29%7D%7B14%5C%2Ccm%7D-5%5C%2Ccm%2B3%5C%2Ccm%5C%5C5x%3D%5Cfrac%7B49%5C%2Ccm%5E%7B2%7D%7D%7B7%5C%2Ccm%7D-2%5C%2Ccm%5C%5C5x%3D7%5C%2Ccm-2%5C%2Ccm%5C%5C5x%3D5%5C%2Ccm%5C%5Cx%3D%5Cfrac%7B5%5C%2Ccm%7D%7B5%7D%3D1%5C%2Ccm" />

Finalmente se tiene :

<img src="https://tex.z-dn.net/?f=b%3D5x-3%5C%2Ccm%3D5%281%5C%2Ccm%29-3%5C%2Ccm%3D5%5C%2Ccm-3%5C%2Ccm%3D2%5C%2Ccm%5C%5CA%3D%5Cfrac%7B%285%5C%2Ccm%2B2%5C%2Ccm%29%2814%5C%2Ccm%29%7D%7B2%7D%3D%5C%5CA%3D%5Cfrac%7B%287%5C%2Ccm%29%2814%5C%2Ccm%29%7D%7B2%7D%5C%5CA%3D%5Cfrac%7B98%5C%2Ccm%5E%7B2%7D%7D%7B2%7D%3D49%5C%2Ccm%5E%7B2%7D%5C%5C49%5C%2Ccm%5E%7B2%7D%3D49%5C%2Ccm%5E%7B2%7D" />

Saludos.