2.5-7.5t=-4.9t^2

Solution for 2.5-7.5t=-4.9t^2 equation:

2.5-7.5t=-4.9t^2

We move all terms to the left:

2.5-7.5t-(-4.9t^2)=0

We get rid of parentheses

4.9t^2-7.5t+2.5=0

a = 4.9; b = -7.5; c = +2.5;
Δ = b2-4ac
Δ = -7.52-4·4.9·2.5
Δ = 7.25
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{-(-7.5)-\sqrt{7.25}}{2*4.9}=\frac{7.5-\sqrt{7.25}}{9.8}
$
$t_{2}=\frac{-b+\sqrt{\Delta}}{2a}=\frac{-(-7.5)+\sqrt{7.25}}{2*4.9}=\frac{7.5+\sqrt{7.25}}{9.8}
$