u/Aristotle04

So I tried solving this system of equations (pic 1) using substitution (pic 2), but when I plugged in my result, I found that my answer can’t be right. I know I made a mistake, but I don’t know where, so can anyone help out? With maybe a suggestion of the right way to do it?

Sorry, I’m sure I’ve made a juvenile mistake here, but it’s been a while since I took algebra so I’m just looking for a little illumination on how to go about this.