What specific narrative style does updike use in "the slump"?

I would say that the style of narration in "The Slump" is specific because there are no dialogues - the entire story is told in the form of a monologue, inside the protagonist's head. We see that the narrator is telling the story as he is experiencing it in his own mind, and Updike probably used a technique such as 'stream of consciousness' to convey his rambling thoughts. 

substitute y = 2x - 4 to x + 2y = 12:

x + 2(2x - 4) = 12     use distributive property

x + (2)(2x) + () = 12

x + 4x - 8 = 12     add 8 to both sides

5x = 20     divide both sides by 5

x = 4

put the value of x to the first equation:

y = 2(4) - 4

y = 8 - 4

y = 4

answer: x = 4 and y = 4