the tangent line (or simply tangent) to a plane curve at a given point is the straight line that "just touches" the curve at that point. Leibniz defined it as the line through a pair of infinitely close points on the curve.
The reason why c = 6n + t is the same as c - t = 6n is because c is the sum of the addition problem. 6n and t are the addends. The inverse operation for addition is subtraction. c is the cost that will be reduced by t. The answer is still 6n.
