Integrating gene expression with summary association statistics to identify genes associated with 30 complex traits

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 believe it is "they are not used up by reactions" as the answer to your question, text me back if this is correct because i just learned this.