A net force on a fluid or wall comes from the pressure difference across it, not from the absolute pressure level. Flow and net push go from high to low pressure, with magnitude set by delta-P times area. The one-way result holds for the stated boundaries and ignores viscosity and turbulence, which real fluids add back in.
A pressure difference gives a checked one-dimensional net force. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
A pressure difference is checked first
Left pressure 8 pascals minus right 2 pascals gives difference 6 pascals.
8minus2=6
The difference times area gives the net push
Difference 6 pascals over area 3 square metres gives net force 18 newtons to the right.