In this case we have:
3x = -7x^2+9
so subtracting 3x from both sides we get:
0 = -7x^2 - 3x + 9
or subtracting -7x^2+9 from both sides we get:
7x^2 + 3x - 9 = 0.
These two can then be transformed into one another by switching
the equlities around and multiplying by -1.
Note: 7x^2 + 3x - 9 = 0 is the same thing as 0 = 7x^2 + 3x - 9.