17. $$5^{log_7 98 \over log_7 2}$$

17. $$5^{\frac{log_7 98}{log_7 2}} = 5^{log_2 98} = 5^{log_2 (2*49)} = 5^{log_2 2 + log_2 49} = 5^{1 + log_2 7^2} = 5^{1+2log_2 7}$$. Ответ: $$5^{1+2log_2 7}$$