.
You substitute y = 7x-42 from the first equation into the second equation.
Doing this way, you obtain a quadratic equation for x.
You solve it and obtain the solutions for x (if they do exist).
Then for each found value of x you find the corresponding value of y, using the first equation.
Having my instructions, the rest is standard mechanical work, which I leave to you.
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To see many other similar solved problems (yours TEMPLATES), look into the lesson
- Solving systems of algebraic equations of degree 2 and degree 1
in this site.