Here are some possible suggestions. I hauled the addresses out of our reference material sticky topic :)
z score probabilities - this is great
http://davidmlane.com/hyperstat/z_table.html
Another Stats calculator - More versatile than the first (Teach yourself Stats site!)
http://stattrek.com/online-calculator/normal.aspx
y= -.04x^2+1.2x.....I assume you want to find the roots of this polynomial
First....set it to 0
-.04x^2+1.2x = 0 multiply through by 100 to clear the decimals
-4x^2 + 120x = 0 factor
-4x( x - 30) = 0 set each factor to 0
-4x = 0 divide by-4 on both sides and x = 0
x - 30 = 0 add 30 to both sides and x = 30
And those are the two roots......here's a graph....https://www.desmos.com/calculator/uwcv2ac06n