We can calculate this value using property of exponentiation in modular arithmetic
$If \ a \equiv b(modN), then\ a^{k} \equiv b^{k}(modN) \ for \ any \ positive \ integer$ k
$17^{8}(mod47) \\ \equiv (17^{2})^{4}(mod47) \\ \equiv 7^{4}(mod)47 \ \because 289mod47=7 \\ \equiv (7^{2})^{2}(mod47) \\ \equiv 2^{2}(mod47) \ \because \ 49mod47=2 \\ \equiv 4(mod47) \\ \equiv 4$