second grader homework

[second grade parent]

Hi I am trying to work on a puzzle for my second grader to learn math. I am trying to dupelicate it in excel with rand number generator. Its a subtraction problem and sometimes the second number or third # is bigger than the first. Is there a way to program excel to hit F9 if the result is negative.
its kind of like sudoku:
A1 blank
A2 =(B1+C1)-A3A3 =RANDBETWEEN(6,12) this is supposed to be a1-a2 this is supposed to be a positive number
B1 = RANDBETWEEN(6,10)
B2 blank
b3 =B1-(A2-C2) this is supposed to be B1-B2 and this is supposed to be a positive number
C1 = this is supposed to be a1-b1 this is supposed to be a positive number
c2 this is supposed to be a2- b2 this is supposed to be a positive number.

how should i program this to only result in positive number?

 

[shg]

Its a subtraction problem

That's why the number on the bottom is negative. The results are all non-negative.