Answer
See below
Work Step by Step
Given $S$ be the subspace of $R^3$ consisting of all vectors of the form $v = (c_1, c_2, c_2 − 2c_1)$
Rewrite $(c_1, c_2, c_2 − 2c_1)=c_1(1,0,-2)+c_2(0,1,1)$
Hence, set of vectors $(1,0,-2),(0,1,1)$ spans $S$