91=15t+5t^2

Solution for 91=15t+5t^2 equation:

91=15t+5t^2

We move all terms to the left:

91-(15t+5t^2)=0

We get rid of parentheses

-5t^2-15t+91=0

a = -5; b = -15; c = +91;
Δ = b2-4ac
Δ = -152-4·(-5)·91
Δ = 2045
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$t_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$t_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

$t_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-15)-\sqrt{2045}}{2*-5}=\frac{15-\sqrt{2045}}{-10}
$
$t_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-15)+\sqrt{2045}}{2*-5}=\frac{15+\sqrt{2045}}{-10}
$