You could just subtract it or you could make it into a plus negative
<h2>Number of bacteria after 6 days is 2313</h2>
Step-by-step explanation:
Population after n days is given by

Initial population, P₀ = 1000
Increase rate, r = 15 % = 0.15
Number of days, n = 6
Substituting

Number of bacteria after 6 days = 2313
Slope = y2 - y1 / x2 - x1
= 8-4 / 3-2
= 4 / 1
= 4