Hi xy   
Thanks for answering HSDx and guest   But, guest, my answer is for you to learn from as well
   But, guest, my answer is for you to learn from as well    
 
What real value of t produces the smallest value of the quadratic t^2-9t-36?
maybe it will make more sense to you change the t to an x and then put it equal to y
 
\(y=x^2-9x-36 \)
Now you are being asked for the smallest value of x
     If you graph this you will get a concave up parabola.
    I know this becasue the x^2 makes it a parabola
    and the invisable number in front of the x^2 is  +1. 
    Since it is + the parabola will be concave up.
 
The lowest point of a concave up parabola will be the turning point. The x value for it which will be halfway between the roots.
 
roots are when y=0 so
 
\(0=x^2-9x-36\\ 0=(x-12)(x+3)\\ x=12\;\;or \;\;x=-3 \)
 
Halfway between the roots is  x= (12+-3)/2 = 4.5
 
That is the x value (actually the t value) that you want.     t=4.5
 
that is the value that will give the expression the smallest value.
If you wanted to find that smallest value then you would sub 4=4.5 back into the expression.
 
 
