Imagine that individuals with a certain allele (B) have much greater reproductive success than those without the allele. However, individuals with the B allele also have shorter life spans than those without the allele. How would you predict the allele frequencies in the population will change through subsequent generations

Explanation:

The B allele will become more common only if the individuals reach reproductive ages

Since the individuals with allele B have much greater reproductive success therefore once they reach the reproductive age, success to reproduce new individuals will be higher

The offsprings will carry on the allele(B allele) and pass on to the next generation

Even if the parental individuals die because of shorter life span the offsprings produced in each generation will pass on the allele to their subsequent generation  

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.

: -)