Sum of 3l – 4m – 7n2 and 2l + 5m – 4n2
= 3l – 4m – 7n2 + 2l + 3m – 4n2
= 3l + 2l – 4m + 3m – 7n2 – 4n2
= 5l – m – 11n2 ……………………..equation (1)
Sum of 9l + 2m – 3n2 and -3l + m + 4n2
= 9l + 2m – 3n2 + (-3l + m + 4n2)
= 9l – 3l + 2m + m – 3n2 + 4n2
= 6l + 3m + n2 ……………………….equation (2)
Let us subtract equation (i) from (ii), we get
= 6l + 3m + n2 – (5l – m – 11n2)
= 6l – 5l + 3m + m + n2 + 11n2
= l + 4m + 12n2