Agroup of scientist study the effect of chemical on various strains of bacteria. strain a started with 6000 cells and decreases at a constant rate of 2000 cells per hour after the chemical was applied. strain b started with 2000 cells and decreased at a constant rate of 1000 cells per hour after the chemical was applied. when will just strains have the same number of cells? explain

So
number in a=6000-2000x where x=number of hours

number in b=2000-1000x

when will a=b
so set them equal to each other and solve
6000-2000x=2000-1000x
subtract 6000 from both sdies
-2000x=-4000-1000x
add 1000x to both sdies
-1000x=-4000
multiply -1
1000x=4000
divide by 1000
x=4

in 4 hours, they will be the same, but, their numbers will be negative, so there is actually no real life time when they will be the same observe
1 hour:  a=4000, b=1000
2 hour a=2000 b=0
3 hour  a=0  b=-1000
4 hour a=-2000 b-=-2000
but you can't have -2000 cells, you can only have 0 or more than 0