Technically, this could be a number of things depending on your standing in Math.
For those who are math wizards/experts:
This is a hyperbole. A hyperbole is an exaggerated statement. A hyperbole does not say that the clause is false, it is just overemphasized. Mastering calculus may be easy, but it is nowhere as easy as mastering your ABCs (unless you really are a math prodigy ;D)>
For those are not math wizards/experts:
This is irony. Irony is a figure of speech where you compare something to its opposite and is normally used as sarcasm or mockery. Calculus is known universally for being the hardest math level, so comparing it to something as easy as the ABCs would be called irony.
Hope this helps!