so this is very simple.
if f(x)=12x +50, and we want to know how much the cost goes up per square yard, we look at the relationship.
if the coefficient of x is 12, the cost will go up $12 per square yard.
it is NOT 62 because the 50 is a one time thing that is only added one time. think about it like having the price be $50 at 0 yards. it's almost like a flat fee. for 1 yard its 62, 2 its 74, etc. it goes up 12 each time. additionally, it's not x because x is the amount of yards, not the cost of a yard. it is not 50 because 50 is added, and is not in relationship to x.
so, the answer is b.
hope I helped.