a) By definition, you have that the period of the y=aCos(bx) is:
Period=2π/|b|
b=2π/3
Period=2π/(2π/3)
Period=3
b) The amplitude of y=aCos(bx) is:
Amplitude=|a|
Amplitude=1/4
c) The mideline represents the horintal line that passes between the maximum and minimum points. In the equation y=aCos(bx) the mideline is: y=0