Urgent!! Physics p4 O/N 2006 Qn 6 (b) graph

[gentleman]

Help me finding the period..
i got the peak value in a (i) part to be 8.5 so
2.0Vcm^-1 ----- 1 cm
8.5Vcm^-1 ----- (4.25)
4.25 squares along y axis
please help me for x axis

 

[beacon_of_light]

f=50Hz
So T = 1/50 = 0.02s or 20ms
time base is 5ms/cm
so 20ms = 4cm ...