The sum of x square + my-12 and polynomial nx square-3y + 6 does not contain X and Y. try to find the value of Mn

The sum of x square + my-12 and polynomial nx square-3y + 6 does not contain X and Y. try to find the value of Mn

X square + my-12 + NX square - 3Y + 6 = (n + 1) x square + (M-3) y-6
If x and y are not included, then n + 1 = 0, M-3 = 0, so n = - 1, M = 3;
So Mn = - 3
If you don't understand, please hi me,

Simplify mathematical formula
2 to the power of 1999 + 2 * 2 to the power of 1998 + 3 * 2 to the power of 1997 + +1999*2+2000

Let the sum of them be s, then s = 2 ^ 1999 + 2 * 2 ^ 1998 + 3 * 2 ^ 1997 + +1999*2^1+2000*2^02S=2*2^1999+2*2*2^1998+2*3*2^1997+…… +2*1999*2^1+2*2000=2^2000+2*2^1999+3*2^1998+…… +1999*2^2+2000*2^12S-S=2^2000+2^1999+2^1998+…… ...

What formula can be used to simplify 31x29

Square difference formula
31x29
=(30+1)(30-1)
=30^2-1^2
=900-1
=899