Incorrect equation for I

Hi Aili and Brian,

There was an error in Aili's solution: the current entering the top node is I + i_1 and the current leaving the top node is i_2, so we get I = i_2 - i_1. A circuit very similar to this one also appears in the following problem:

http://wiki.ubc.ca/Science:Math_Exam_Resources/Courses/MATH152/April_2017/Question_B_03_(a)