I need help with this I’m confused

Eric has challenged himself to walk 24,000 steps in 4 days. if eric walks the same number of steps each day, which function represents the number of steps eric still needs to walk to reach his goal with respect to the number of days since he started his challenge? a. y = 8,000x − 24,000 b. y = -8,000x + 24,000 c. y = 6,000x − 24,000 d. y = -6,000x + 24,000

If f(x)= sqrt4x+9+2, which inequality can be used to find the domain of f(x)?

How do i simply this expression (quadratic formula basis) on a ti-84 or normal calculator?