It is known that the square difference of two prime numbers is equal to 21, so what is the sum of the squares of these two prime numbers?
已知两个质数的平方差等于21，那么，这两个质数的平方和等于多少．
<gadget id="calculator">2 ** 2</gadget>
<output>4</output>

<gadget id="calculator">21 + 4</gadget>
<output>25</output>

<gadget id="calculator">25 + 4</gadget>
<output>29</output>

<result>29</result>