Think about the meaning of 0.3. Does it mean 3 tenths? 3 hundreths? 3 thousandths? Which part of 8.42 has the same place value?
0.3 means 3 tenths. 8.42 means 8 wholes, 4 tenths, and 2 hundreths. Can you combine these values now?
Maciel is correct. Can you explain how Alejandro found the wrong sum?