What subway do you take from Penn Station to 70th Street?

Take the Uptown E train (of the A-C-E, the blue line) from 34th Street-Penn Station to Lexington Avenue-53rd Street.

The Lexington Avenue-53rd Street Station is connected to the 51st Street Station on the 6 train (of the 4-5-6, the dark green line).

Transfer to the Uptown 6, and take the Uptown 6 to 68th Street-Hunter College (at Lexington Avenue). Then walk east to York Avenue.