y=7cos(x-pi/6) x = 2pi/3

Question

Not sure how to solve. 

Answer 1

y=7cos(x-[∏/6]) where x=2∏/3

y=7cos(x-30º)

2∏/3=120º

y=7cos(120º-30º)

y=7cos(90º)

y=7*0 because cos90º=0

y=0