SUVAT - Straight forward question... quadratics

Hi Guys,

Here is a pretty straight forward question.

The only problem is, I got confused as to what happens with - and + numbers when they are squared etc...

Could anybody kindly guide me through the following problem and explain step by step what is taking place?

Ok here goes:

**Q. A body is projected vertically upwards from ground level at a speed of 24.5ms-¹. Calculate the length of time that the stone is at least 19.6m above ground level.**

**I understand that if I use SUVAT, s = 19.6, u = 24.5, a = -9.8ms²**

Therefore I must use: s = ut + 1/2 at²

Therefore: 19.6 = 24.5(t) + 1/2 -9.8(t²)

Therefore: 19.6 = 24.5(t) + -4.9(t²)

Therefore: ??? Now i'm unsure how to make t the subject, what happens with the +-4.9 ???