To solve this equation, we will first convert all the mixed numbers into improper fractions.

12 7/23 = (12 * 23 + 7) / 23 = 287/23

6 18/23 = (6 * 23 + 18) / 23 = 156/23

3 21/23 = (3 * 23 + 21) / 23 = 90/23

5 19/23 = (5 * 23 + 19) / 23 = 124/23

Now the equation becomes:

287/23 - (156/23 - t) = 90/23 + 124/23

Simplify the equation:

287/23 - 156/23 + t = 214/23

Add like terms:

131/23 + t = 214/23

Subtract 131/23 from both sides to solve for t:

t = 214/23 - 131/23

t = 83/23

Therefore, t = 3 14/23.