For an increase of 2 crates, the cost increases with $20, so that's $10 per crate. That's our slope. Answers A and C are invalidated immediately.
In the equation, the numbers from one table entry are used. What you do is you want the right hand side to be zero for the used x value, such that the left hand side simplifies to your y value.
In answer B, the entry (8, $85) is used. If you fill in x=8, you want y to be $85 and that's exactly what's happening there.
In answer D, x and y are swapped, so that's very wrong.
A and C were already discarded, which confirms B is the right answer.