Okay so your equation implies that each unit costs a certain amount, A, but also there is some overall associated cost (perhaps startup costs or setup costs or something) which is B.

To my understanding it should be something like this:

TOTAL cost = y = Ax + B

Total Fixed costs = B (this value is the same no matter how many units are made)

Total Variable costs = Ax (this value varies depending on how many units are made)

Then the variable cost per unit is just the Total Variable cost divided by the number of units:

VCU = TVC/x = Ax/x = A =

The fixed cost per unit is the total fixed cost divided by the number of units:

FCU = TFC/x = B/x

Seem logical?