Confused about A Level maths past question

I’m confused about the significance of the brackets in this answer. I did 6x - 7 - 5x + 1 = 5x + 1 - 2x + 5 without the brackets, and ended up getting an incorrect answer of x = 1. I can see mathematically why the brackets make a difference, but I’m confused about how you’re meant to know to include them in the first place??