Mark M. answered 04/06/21
Retired Math prof with teaching and tutoring experience in trig.
Point on tangent line = (π/4, sin[4(π/4)]) = (π/4, 0)
Slope of tangent line is the derivative of y = sin(4x) evaluated at x = π/4 (the derivative of sin(4x) is 4cos(4x))
So, slope = 4cos[4(π/4)] = -4
Using the point slope form, equation of tangent line is y - 0 = -4(x - π/4).
Simplify to get y = -4x + π.