Arithmetic is frustrating is an example of PREDICATE POSITION.
There are three positions where an adjective can be placed in a sentence: before a noun, after a noun or in the predicate. These positions are termed attrubutive, appopositive and predicate positions respectively.
An adjective is in a predicate position when it is placed in the predicate of a sentence.
The adjective in the given sentence is 'frustrating'.
