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x^2 + y^2 =49
y^2 -2x = 49
or, we can write -y^2 + 2x = -49
now, add this equation to first equation,
x^2 + 2x = 0
=> x(x+2) = 0
so, x= 0 or x= -2
when x=0, then y = -7 or 7
when x= -2, then y = -sqrt(45) or sqrt(45)