All times. The numbers you are dividing with are quotients.
Answer is 3/72
multiply the denominator and numerator of 3/8 by 9 (27/72)
multiply the denominator and numerator of 3/9 by 8 (24/72)
this is to get the same denominator of 72
then just subtract
27/72 - 24/72 = 3/72
The answer is -1/3, -5/12, 0.66
You have to give them the same denominator so multiply 11/12 by 4 and 3/16 by 3. giving you 44/48 and 9/48. then divied 44 by 9 giving you 4.8888
there for there are 4 whole 3/16 sections of the 11/12 board