lets consider the number to be 2x.
to find out the square of 2x,simply:
1)always start with the number 400
2)then multiply 40 with your one's digit(we have taken it as 'x')
now, add both..400+40x
3)add x^2 to 400+40x
so, the answer of 2x^2 is 400+40x+x^2