Given:
y=3x-5
21x - 35 = 7y
Substitute the first equation into the second equation.
21x - 35 = 7(3x-5)
Open the parenthesis
21x - 35 = 21x - 35
Collect like term
21x - 21x = -35 + 35
0 = 0
Since, the above statement is true, then it has infinitely many solutions.
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