70=40t-4.9t^2

Solution for 70=40t-4.9t^2 equation:

70=40t-4.9t^2

We move all terms to the left:

70-(40t-4.9t^2)=0

We get rid of parentheses

4.9t^2-40t+70=0

a = 4.9; b = -40; c = +70;
Δ = b2-4ac
Δ = -402-4·4.9·70
Δ = 228
The delta value is higher than zero, so the equation has two solutions
We use following formulas to calculate our solutions:
$t_{1}=\frac{-b-\sqrt{\Delta}}{2a}$
$t_{2}=\frac{-b+\sqrt{\Delta}}{2a}$

The end solution:

$\sqrt{\Delta}=\sqrt{228}=\sqrt{4*57}=\sqrt{4}*\sqrt{57}=2\sqrt{57}$
$t_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-40)-2\sqrt{57}}{2*4.9}=\frac{40-2\sqrt{57}}{9.8}
$
$t_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-40)+2\sqrt{57}}{2*4.9}=\frac{40+2\sqrt{57}}{9.8}
$