Important: If you're discussing squares, please express "the square of x" as x^2 (not as x2).
Please note that your y=x^2+3 is NOT a line. I will assume (probably incorrectly!) that you meant y=2x+3. The slope of this line is 2, so the slope of any line perpendicular to y=2x+3 is -1/2.
Now use the point-slope equation of a straight line:
