|I dont know what you are trying to do but there a few obviouse errors.|
You have Return SingleRoll = area / coverage in the singleRoll function which is fine.
You also have Return SingleRoll(area, coverage) in the btnCalc_Click function which is not correct notwhichstanding that you cannot return the product of a function from outside the function.
You are also attempting to return a value from a subroutine which is not allowed.
If you are attempting to call the SingleRoll function from the btnCalc_Click subroutine cut out the return.
Nowhere in the btnCalc_click function are the variables given values before you start doing calculations on them.
perimeter = 2 * length + 2 * Width is meaningless becasue they will always be zero as will height.