For 2 million coins at $9.75 each you paid $9.75(2,000,000) = $19,500,000

For 1 million coins at $8.00 each you paid $8.00(1,000,000) = $8,000,000

Hence you paid a total of $19,500,000 + $8,000,000 for 3 million coins and thus your average cost per coin was ($19,500,000 + $8,000,000)/3,000,000