math help please?

the range of the function
f(x)=-2cos(3(x-pi))+4 is

a. 2 =< f(x) =< 6
b. 6 =< f(x) < 10
c. -10 =< f(x) =< - 6
d. -6 =< f(x) =< -2
which is it?
please provide detail calculation. thank you