Answer:
11
Step-by-step explanation:
Substitute 3 for b
23 - 4(3)
Multiply 4(3) = 12
23 - 12
Subtract
23 - 12 = 11
So, let's make these two values improper fractions so they're easier to work with. 
So, to get the rate he grew in those months we have:
And to get the rate he grew in one month we have to divide both our numerator and our denominator by 21 to get:
aka he grew 1.24 inches in a month.
120138 subtract both and that would be the answer
804 * 219 = 176,076
3674 * 89 = 326,986
4321 * 96 = 414,816
6782 divided by 5 = 1356.4
3224 divided by 96 = 33.58
3224 divided by 8 = 403