You would start by finding the area of the circle and then finding the area of the rectangle. After finding both, subtract the area of the rectangle from that of the circle. <span />

We know that,
- Pythagoras theoram is given by : p² + b² = h²
- In a Pythagoras triplet, the longest side represents the hypotenuse of the triangle.
Then,
We should check that, whether the sum of squares of the smaller sides will be equal to the square of the third side or not. Then, we can determine the above easily.
Sum of squares of smaller sides:
= 6² + 8²
= 36 + 64
= 100
Square of longer side:
= 10²
= 100
Sum of squares of smaller sides = Square of longer side. Hence, 6,8,10 are Pythagorean triplets.
And we are done! :D
⛱️ 
Next term will be 100
as series is 18²,16²,14²,12² so next will be 10²
hope it helps