Question

the difference between the heights of your chair and your desk is -10 1/4 inches. the height of your desk is 29 3/4 inches. what is the height of your chair?

Answer 1

Since you said the height of your chair and you desk, chair would go first when subtracting.
C= chair
c - 29.75 = -10.25
 + 29.75  = + 29.75
c= 19.5
So, you chair is 19.5 (19 1/2) inches tall.