Need some help on these final 4 problems thanks.

-5 -4-3-2-1 (6) Find critical numbers for f(t) = vt(1 - t) where t > 0. What will you do first? (7) Find the maximum and minimum values of f(x) = 3x + 1 defined on the interval [3, 6]. (8) Find the maximum and minimum values of f(x) = x2 + 4 defined on the interval [-2, 6]. (9) Find the maximum and minimum values of f(x) = sin(2x) + cos(2x) on the interval [-7/4, 7/2]. Then graph the function to check your answers