Cassidy's diving platform is 6 ft above the water. One of her dives can be modelled by the equation d = x2 - 7x + 6, where d is her position
relative to the surface of the water and x is her horizontal distance from
the platform. Both distances are measured in feet. How deep did Cassidy
go before coming back up to the surface?

Before coming back up to the surface the maximum depth, Cassidy went was 6.25 ft. below the water surface

Step-by-step explanation:

The height of Cassidy's diving platform above the water = 6 ft.

The equation that models her dive is d = x² - 7·x + 6

Where;

d = Her vertical position or distance from the water surface

x = Here horizontal distance from the platform

At Cassidy's maximum depth, we have;

dd/dx = d(x² - 7·x + 6)/dx = 2·x - 7 = 0

x = 7/2 = 3.5

∴ At Cassidy's maximum depth, x = 3.5 ft.

The maximum depth, = d(3.5) = 3.5² - 7 × 3.5 + 6 = -6.25

The maximum depth, Cassidy went before coming back up to the surface = = -6.25 ft = 6.25 ft. below the surface of the water.