Mass oxigen = 2.81 - 2.50 = 0.31 g
Moles O = 0.31 g / 15.9994 g/mol =0.0194
moles Cu = 2.50 g / 63.546 g/mol =0.0393
we divide by the smallest number
0.0194 / 0.0194 = 1 => O
0.0393 / 0.0194 = 2 => Cu
the formula is Cu2O ( Copper (I) oxide )
Ya got this