The first equation can be rewritten as 2x - y = - 6. I will then substitute the second equation in where the y-variable is. This give you the equation:
2x - (x - 1) = - 6.
Distribute the negative to both terms in the parentheses.
1st month you spent $300. And the flight back is $200. 300+200=500. 1500-500=1000. Yoy have 1000 dollars to spend in 4 months. 1000/4=250, so each month you can spend $250.