Answer
$r_s\lt -0.446$: null hypothesis is rejected.
There is enough evidence to conclude that engine displacement and fuel economy are negatively associated.
Work Step by Step
$H_0: X~and~Y~are~not~associated$ versus $H_1: X~and~Y~are~negatively~associated$
See the picture.
$∑d_i^2=42.25+49+182.25+132.25+2.25+42.25+25+0.25+2.25+169+72.25+144+16+132.25+42.25=1053.5$
$r_s=1-\frac{6∑d_i^2}{n(n^2-1)}=1-\frac{6\times1053.5}{15(15^2-1)}=-0.881$
Critical value = -0.446
(According to Table XIV, for n = 15 and α(1) = 0.1)
Since $r_s\lt -0.446$, we reject the null hypothesis.
