Answer
So the equation $x_1v_1+x_2v_2+x_3v_3=b$ has at least two solutions, not just one solution. (In fact, the equation has infinitely many solutions.)
Work Step by Step
See the parallelograms drawn on the figure from the text that accompanies this exercise. Here c1, c2, c3, and c4 are suitable scalars. The darker parallelogram shows that b is a linear combination of v1 and v2, that is
$c_1v_1+c_2v_2+0\cdot v_3=b$
The larger parallelogram shows that b is a linear combination of v1 and v3, that is,
$c_4v_1+0\cdot v_2+c_3v_3=b$
So the equation$ \ x_1v_1+x_2v_2+x_3v_3=b\ $ has at least two solutions, not just one solution. (In fact, the equation has infinitely many solutions.)
