Consider the circle
we have the intersecting chords theorem, which states that

In our case we have a=x, b=12, c=6 and d=x+4. So we have

distributing on the right side we get

Subtracting 6x on both sides, we get

Dividing boht sides by 6, we get

So, the value of x is 4. Now we replace this value to find the length of each chord, so we have
x---->4
12---->12
x+4----->4+4=8
6----->6