Oc is perpendicular to ab. what is the length of ab

I have attached the diagram associated with this question.

AB = 7.4 units

Explanation:
The perpendicular distance from the center of the circle to any chord in the circle bisects this chord.

In the given:
O is the center of the circle
AB is a chord in the circle
OC is perpendicular on AB

This means that:
OC bisects AB
AC = CB

We are given that:
AC = 3.6 units

Therefore:
AB = AC + CB
AB = 3.6 + 3.6
AB = 7.2 units 

Hope this helps :)