The answer is:
v = 15/T sin t/T
then integrate
x = -15 cos t/T + c
if x = 4 at t = 0 then
4 = -15 + c so c = 19
so
x = 19 - 15 cos t/T
at t = 4T
x = 19 - 15 cos 4 = 3.99 or 4
b)
a = dv/dt = 15/T^2 cos t/T
v(0) = 0 since sin 0/T = 0
a(0) = 15/T^2
so
6 + 0 +(7.5/T^2) (16 T^2)
= 6 + 7.5*16
= 126 is the answer