The standard form is known to be
ax^2 + bx + c = 0
You were given 7x^2= 12x.
Subtract 12x from both sides.
7x^2 - 12x = 0
We can easily see that a = 7, b = -12 and c must be 0.
See below:
7x^2 - 12x = 0 is exactly the same as
7x^2 - 12x + 0 = 0.
So, c = 0.