Answer
a) period is $\pi/156$
b) maximum brightness is 10 and the minimum is 5.8
Work Step by Step
a) the period is found as the variable $k$ in the base function $f(t)=a\cdot cos(k(t-b))$ In this case one can see that it is $\pi$/156
b) The maximum brightness is determined by the vertical shift, 7.9 in this case, plus the amplitude, 2.1 in this case. $7.9+2.1=10$
The minimum brightness is determined by the vertical shift, 7.9 in this case, minus the amplitude, 2.1 in this case. $7.9-2.1=5.8$