Answer
$c_{y}=80.0$ 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 $y$ axis we have
$(40.0-70.0+c_{y}-70.0)$ cm$=-20.0$ cm
$(c_{y}-100.0)$ cm$=-20.0$ cm
$c_{y}=80.0$ cm.
