Find I_1 in the circuit.
Image from: Irwin, J. David., and R. M. Nelms. Basic Engineering Circuit Analysis, Tenth Edition. N.p.: John Wiley & Sons, 2010.
First, we will pick two nodes to write Kirchhoff’s current law around them (Forgot Kirchhoff’s current law?). They are labeled A and B in the diagram and were arbitrarily picked. We also labeled I_2 as current leaving node A towards node B, which is also labeled in the diagram.
(negative because the assumption we made of current leaving node A is wrong, in fact, the current is entering node A in accordance with passive sign convention)
This question can be found in Basic Engineering Circuit Analysis, 10th edition, chapter 2, question 2.13.