Momentum is determined by:
a. speed and weight
b. speed and size
c. mass and velocity
d. size and weight

try using substitution:

x + y - 6 = x - y. (i'm not giving you the exact answer). since 0 = x - y, then sub 0 in the equation with x - y. combine your like terms, and then use inverse operations, and you'll have your answer.

: -)

i think it is dna, i'm not totally sure but i think it might be.