You plug in the x = 3y+4 into the 3x part of 3x -5y=8. You distribute the 3 and add like terms to end up with 4y=12. You divide by 4 to get y alone and get -1. That’s your y value. You plug that -1 into the y part of any of the two equations to find x. The simplest one is to plug it in to x=3y+4, and you get -3+4=x, so x= -1
