We can see that the difference is 8 so we can write:

X = 35
use the formula a^2 + b^2 = c^2 in order to find the hypotenuse, which is x.
21^2 + 28^2 = c^2
441 + 784 = c^2
1225 = c^2
35 = c
x = 35
Answer:
b=7.5
Step-by-step explanation:
The number of bacteria that will exist in 12 hours is 614,400.