Please help me answer this calculus questions.

Question

Determine the points on the curve y = 4x- tan x where slope of the tangent is 0 in the interval 0 ≤ x ≤ π.

Answer 1

Answer 2

Set derivative to 0:

4-sec²x = 0

(1/cos²x) = 4

cos²x = 1/4 => cos x = ±1/2 => x= 30° or 150°.