The radius of a circle is the distance from the center point of a circle to the outer edge of the circle. The diameter is the distance between one point on the outside of a circle and a point on the other side, and the diameter passes through the center point.
Therefore, the diameter is just twice the radius.
We know the radius is 3.5 cm, so we can just multiply:
3.5 * 2 = 7
The diameter of a circle with a radius of 3.5 cm is 7 cm.
Hope that helped! =)