book Maths Puzzle Numerical Ability

I need help on solwing this linear equation by substitution..
3x-y=7
y=x+3

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Okay, so I'm not going to get this math without some serious help. I do not understand this. Could someone please help me before I have a total meltdown? This is my issue that I wo I have looked in so many places to find a solution or method for this, and I have not succeeded. I would appreciate some help!

f(x) = 0

f(x) = x^2 + 2x - 1