find x intercept;
y=-0.01x^2+0.7x+6.1
The x-intercept on a quadratic expression is essentially solving for the roots of a quadratic equation.
For the above, the intercepts can be found by solving for x when y=0, i.e.
-0.01x^2+0.7x+6.1 = 0
You can solve by completing the squares, or by the quadratic equation.
Either case, the sum of the two roots should be 70.