21=-16t^2+26t+11

Solution for 21=-16t^2+26t+11 equation:

21=-16t^2+26t+11

We move all terms to the left:

21-(-16t^2+26t+11)=0

We get rid of parentheses

16t^2-26t-11+21=0

We add all the numbers together, and all the variables

16t^2-26t+10=0

a = 16; b = -26; c = +10;
Δ = b2-4ac
Δ = -262-4·16·10
Δ = 36
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$t_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$t_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$\sqrt{\Delta}=\sqrt{36}=6$
$t_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-26)-6}{2*16}=\frac{20}{32}
=5/8
$
$t_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-26)+6}{2*16}=\frac{32}{32}
=1
$