Twice a number, x, divided. y 6 is 42

The graph below shows the amount of money left in the school’s desk fund, f, after d desks have been purchased. for each new desk that is purchased, by how much does the amount of money left in the school’s desk fund decrease?

What is the simplified expression for 6(2(y+x))

Aline on the wall and a line on the floor are skew. always, sometimes, or never?