1,3,8,21,55 () () fill in the numbers according to the rules. How can the third grade explain to the children

1,3,8,21,55 () () fill in the numbers according to the rules. How can the third grade explain to the children

Multiply a number by 3 and subtract the number before it = the number after it
1、3、8、21、55、144、377
The latter number multiplied by 3 minus the former number equals the next number
3*3-1=8
8*3-3=21
21*3-8=55
55*3-21=()=144
144*3-55=()=377
The two numbers in () are 144377