Oh those were the days!
Think of it like this: (^ is notation for to the power of)
x^2 + 3/10 - 6/5 = f(x)
You need two numbers that when mutiplied by each other give 6/5 (or 1.2) and then add together to equal 3/10 (or 0.3).
A quick check shows it's not so simple as your example. I used my supercalc and found straight away that I was correct.
The actual answers are (x-0.9556) and (x+1.255) (check it and see)
Now I wish you look trying to work that out instantly, I couldn't.
You now have two choices, CTS (complete the square) or use quadration equation. I will assume you haven't done much CTS so the equation it is:
x = -b + or - RT(b^2 - 4*a*c)/2a
where:
ax^2 + bx + c = 0 (you can prove it works too, one of the easiest proofs but not necessary). This will give the values at which the curve crosses the x-axis, i.e. the value of x that gives y as zero.
Hence:
-0.3+ or - RT(0.3^2 - 4*1*-1.2)/2x1
=> -0.3 + or - RT(0.09 + 4.8)/2
So -0.3+ or - RT( 4.89)/2 gives the ROOTS of the equation when it is equal to zero.
Hence x = 0.9556672.....
or x = -1.2556672.....
However, if you have understood this, these are the values that would give your sum as being 0, but you don't have an equation like that, hence you reverse the sign (you have worked backwards):
=> f(x) = (x+1.226)(x-0.956) (all to 3sf)
CTS would require me to have a piece of paper and a moment of thought.
Hope that helps, if not, let me know.
