for the value of difference to be largest, the minuend should be maximum(most possibly) and the subtrahend should be minimum
[in A-B=X, A is minuend and B is subtrahend ]
so, $a.b should be maximum. as there is a condition that 4 digits should be distinct, the product will be maximum if we choose 2 maximum valued numbers from the given numbers. so, one of them should be 9 and the other should be 8.
therefore, $a.b=9*8=72
as mentioned above, c.d$ should be minimum. this will be possible only when we choose 2 minimum valued numbers from the given numbers. so, one of them should be 1 and the other should be 2.
therefore, c.d$ = 1*2 = 2
hence, the difference = 72-2 = 70
thus, the largest possible value of the difference $a.b - c.d$ = 70
