I1+I2=6mA how I don't understand

Let's break down why this might be the case:

• KCL at a Junction: Imagine a point in the circuit where three wires meet. KCL states that the total current flowing into that point must equal the total current flowing out of it.
• Possible Scenario: If $I_1$ and $I_2$ are currents entering a junction, and $6 \text{ mA}$ is the total current leaving that same junction, then KCL would give you the equation $I_1 + I_2 = 6 \text{ mA}$.

Without seeing the circuit diagram, it's hard to be certain, but this is the most common reason for such an equation to appear.

Do you have the circuit diagram you can share? That would help me explain exactly where this equation comes from.