My friend needs explaining this and i don't know anything.

To solve this equation (correct me if i'm wrong), you could first multiply the bottom equation by 3, to get 15y. After that, we get -4x-15y=-17 and -3x + 15y=-39, and if we add the 2 equations up, on top of each other, we get -7x=-56, and we can conclude that x=8, and with substitution (-32 - 15y = -17), we know that y=(-1)

Here's what I mean by on-top

-4x-15y=-17

-3x + 15y=-39

-7x=-56

b. astronomers