2 3/12 +3/x = 2 5/8
simplify to
9x+12/4x = 21/8
Cross multiply
(9x+12)/(4x) = 21/8
(9x+12)*(8) = 21*4x
72x+96 = 84x
Subtract 84 x from both side
72x+96 - 84x = 84x -84x
-12x+96 = 0
Subtract 96 from both sides
-12x+96-96 = 0-96
-12x = -96
Divide both sides by 12
-12x/-12 = -96/-12
x = 8
The missing value is 8