Question: how to calculate two unknowns?
Answer: Example 1: 2x + y = 1. -3x + 2y = 0. Solve one equation for a single variable (which one? ... 
Example 2: 2x + y = 1. -2x - y = 2. Solve the first for y: ... 
Example 3: 2x + y = 1. 6x + 3y = 3. Solve the first for y: