What is the relationship between boiling point and pressure?

The boiling point of a liquid is the temperature at which its vapor pressure is equal to the pressure of the gas above it. The normal boiling point of a liquid is the temperature at which its vapor pressure is equal to one atmosphere. Microscopic view inside a bubble in boiling water.

it has one origin of replication, but it proceeds in two directions - d)

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.

: -)