25=42t-4.9t^2

Solution for 25=42t-4.9t^2 equation:

25=42t-4.9t^2

We move all terms to the left:

25-(42t-4.9t^2)=0

We get rid of parentheses

4.9t^2-42t+25=0

a = 4.9; b = -42; c = +25;
Δ = b2-4ac
Δ = -422-4·4.9·25
Δ = 1274
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:

$\sqrt{\Delta}=\sqrt{1274}=\sqrt{49*26}=\sqrt{49}*\sqrt{26}=7\sqrt{26}$
$t_{1}=\frac{-b-\sqrt{\Delta}}{2a}=\frac{-(-42)-7\sqrt{26}}{2*4.9}=\frac{42-7\sqrt{26}}{9.8}
$
$t_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-42)+7\sqrt{26}}{2*4.9}=\frac{42+7\sqrt{26}}{9.8}
$