"Vary directly" is another way of saying "directly proportional".
In simple English, this just means, as one increases, the other increases too.
a = 6
b = 4
The question says find out what a is when b is 16.
First, lets find how many times larger b has gotten.
16/4 = 4
b had gotten 4 times bigger to go from b=4 to b=16
If a is varying directly, it means that it will also get 4 times bigger.
Therefore, a = 6 x 4 = 24