5t^2 - 10t + 4 = 0
t^2 - 2t + 4/5 = 0
There's a -2 on the t in the middle. Half of -2 is -1. -1 squared is 1.
t^2 - 2t + 1 - 1 + 4/5 = 0
(t - 1)^2 - 1 + 4/5 = 0
(t - 1)^2 - 5/5 + 4/5 = 0
(t - 1)^2 - 1/5 = 0
(t - 1)^2 = 1/5
t - 1 = +- sqrt(1/5)
t = +- 1/sqrt(5) + 1
t = +- sqrt(5)/5 + 1
t = -sqrt(5)/5 + 1 or t = sqrt(5)/5 + 1