Concept:
The pressure at the opening is due to both the water column and the oil column.
Equivalent water head is
\[
h_{eq}
=
h_w+\frac{\rho_o}{\rho_w}h_o.
\]
Efflux velocity:
\[
v=\sqrt{2gh_{eq}}.
\]
Step 1: Compute equivalent water head.
\[
h_w=1.4m,
\qquad
h_o=2m.
\]
\[
\rho_o=900,
\qquad
\rho_w=1000.
\]
\[
h_{eq}
=
1.4+\frac{900}{1000}(2).
\]
\[
=
1.4+1.8.
\]
\[
=3.2m.
\]
Step 2: Apply Torricelli's theorem.
\[
v=\sqrt{2gh}.
\]
\[
=\sqrt{2\times10\times3.2}.
\]
\[
=\sqrt{64}.
\]
\[
=8\,ms^{-1}.
\]
Hence
\[
\boxed{8\,ms^{-1}}.
\]