Didn't you mean A = (1/2)d1d2 ?
If so, clear out the fraction by mult. both sides of the eqn by 2:
2A = d1*d2
Now isolate d1 by div. both sides of this result by d2:
2A
------ = d1 (answer)
d2
A = 1/2 d1 d2
divide both sides of the formula by 1/2 d2:-
A / (1/2 d2) = d1 now, using 1 / 1/2 = 2 we have:-
d1 = 2A / d2 answer