The first point is that you ...

The first point is that you have used 'mm' for rainfall and 'cm' for evaporation. It is never a good idea to mix units. Planes have crashed and space missions have failed for this reason.

You don't say which month- so let us assume 31 days.

The inflow is 6.0 m3/s and the outflow is 6.5 m3/s, So the net outflow rate is 0.5 m3/s. The volume of outflow is 0.5 (m3/s) * 31 (days) * 86400 (s/day). That is 1,339,200 m3.

The area of the lake is 50,000 ha, That is 50,000 (ha) * 10,000 (m2/ha) = 500,000,000 m3.

The drop in lake level is 1,339,200 (m3) / 500,000,000 (m2) = 0.002678 m = 2.678 mm. (which can rounded to 2.7 mm)

The change in level is +145 mm (rainfall) - 61 mm (evaporation) - 2.7 mm (net outflow) = 81.3 mm

The final level is 103.20 (m) + 81.3 (mm)/1000 (mm/m) = 103.281 m