2 cm
Step-by-step explanation:
Data
Time (min) Â Â Depth (cm)
2 Â Â Â Â Â Â Â Â Â Â 8
4 Â Â Â Â Â Â Â Â Â Â 15
6 Â Â Â Â Â Â Â Â Â Â 29
8 Â Â Â Â Â Â Â Â Â Â 37
10 Â Â Â Â Â Â Â Â Â 39
12 Â Â Â Â Â Â Â Â Â 49
14 Â Â Â Â Â Â Â Â Â Â 55
The  line of best fit to find the amount of water is:
y = 3.9107x + 1.8571
(I used Excel to get it, you can use a calculator or similar programs if you want to)
In the equation, y represents depth (in cm) and x represents time (in min).
The amount of water, in cm, present  initially in the bathtub is found at time equal to zero. Replacing x = 0 in the equation, we get:
y = 3.9107(0) + 1.8571
y = 1.8571  ≈ 2 cm