Answer
$b_{x}=-70.0$ cm
Work Step by Step
Each dart represents a vector, and the sum of all of them represents ($-140\hat{i}-20.0\hat{j}$).
Along the $x$ axis, we have
($30.0+b_{x}-20.0-80.0$) cm$=-140$ cm,
($b_{x}+70.0$) cm$=-140$ cm,
$b_{x}=-70.0$ cm.
