If the ${17}^{\mathrm{th}}$ and the ${18}^{\mathrm{th}}$ terms in the expansion of ${(2 + \mathrm{a})}^{50}$ are equal, then the coefficient of ${\mathrm{x}}^{35}$ in the expansion of ${(\mathrm{a} + \mathrm{x})}^{-2}$ is