Answer
$b_{x}=-80$ m
Work Step by Step
The individual moves are (meters understood as units)
$\vec{a}=20\vec{i} + 60\vec{j},$
$\vec{b}=b_{x}\vec{i} - 70\vec{j},$
$\vec{c}=-20\vec{i} + c_{y}\vec{j},$
$\vec{d}=-60\vec{i} -70\vec{j},$
The resultant, $\vec{r}=\vec{a}+\vec{b}+\vec{c}=-140\vec{i}+30\vec{j}$
So, (with meters understood as units)
$20+b_{x}-20-60=-140$
$b_{x}=-140-20+60+60=-80$ m
