I think that answer is wrong

The tangent to the equation will have an equation in the form y = mx + yin

(where yin= yitnercept, havent called it c as you already have c defined)

therefore tangent equation is y = (2ax+b)x + yin

you also have an equation y = ax^2 + bx + c

you know these intercept so you can equate them, ie.

ax^2 + bx + c = (2ax +b)x + yin

then solve for yin

Calypso