The question is a bit unclear (i.e., not sure if numerator on the right should be (t-u) or just u...also not sure if the denominator on the right should be du or just d), but as near as I can tell, the solution would be:

A/2 = (t-u)/(du)

A/2 = t/(du)-u/(du)

A/2 = t/(du)-1/d

A/2+1/d = t/(du)

(dA)/2+1 = t/u

(dA+2)/2 = t/u

(dA+2)/(2t) = 1/u

(2t)/(dA+2) = u

Please message me if you'd like to discuss further. Also please best my answer. Thanks.