Answer
$-20\;\rm V$
Work Step by Step
According to Kirchhoff’s loop law,
$$\Delta V_{\rm loop}=\sum_i (\Delta V)_i=0$$
So for our given loop, we know that
$$\Delta V_{12}+\Delta V_{23}+\Delta V_{34}+\Delta V_{41}=0$$
Plug the known;
$$30+50+\Delta V_{34}-60=0$$
Thus,
$$ \Delta V_{34}=\color{red}{\bf-20}\;\rm V$$
