The centroid G′′(α,β) of △PQR be image of centroid of given triangle P′Q′R′.
Centroid of ΔP′Q′R′=(31+3+2,33+1+4)= G′(2,38). Image of G(2,38), w.r.t. line x+2y=2 is (α,β) Then 1α−2=2β−38=1+4−2(2+316−2) ∴1α−2=2β−38=−1532 ∴α=−152 and β=−58 Then 15(α−β)=15(−152+1524)=22