A half-life is how long it takes for half of an element's particles to decay.
In 0 half-lives there will be 100% or 4/4 of the element left
After 1 half-life there will be 50% or 2/4 of the element's original particles left
After 2 half lives there will be 25% or 1/4 of the element's original particles left.
Since there is only 1/4 the original amount of the radium-226, we know that 2 half-lives have passed. Simply divide 3200 by 2 to get the time of 1 half-life.
3200/2 = 1,600 years is the half life of radium-226.
I really hope this helps you =)