COSa=-√5/5 ,tanβ=1/3
tanβ=1/3,3sinβ=cosβ ,9sin^2β=cos^2β
9-9cos^2β=cos^2β ,cosβ=3√10/10
sinβ=√10/10 ,sina=-2√5/5
sin(a-β)=sinacosβ+cosasinβ
=-2√5/5*3√10/10+(-√5/5)*√10/10
=-30√2/50-5√2/50=-35√2/50=-7√2/10
a-β=278.2度