f is one if either f3 is 0 or f1&f2 both are one..
since f2=(4,5,6,7,8,9,10,11,13)
here only 4,5,8.10 are common minterms that means maximally these minterms can participate to make f =1 by (f1&f2) both.,becoz it is AND gate
therefore f3' has to be one one 1,12,14,15 )
since answer is given in maxterms hence it is f3=M(1,12,14,15).D(4,5,8,10)