Aculture initially contains 200 bacteria. if the number of bacteria doubles every hour how many bacteria will be in the culture qt the end of 12 hours

Using the formula for an exponential function: 
y=b*a^x

Insert you values: 
y=200*2^x

b=200 because there are 200 bacteria in the beginning
a=2 because 200%/100%=2. (200 bacause it doubles)

Insert 20 in x and solve for y: 
y=200*2^13=819200

So after 12 hours there are 819200 bacteria. 

I hope that helped.