M1 easy question

A particle is moving along the x-axis with constant deceleration 5m s−2. At time t=0, the particle passes through the origin O with velocity 12m s−1 in the positive direction. At time t seconds the particle passes through the point A with x-coordinate 8. Find (a) the values of t, (b) the velocity of the particle as it passes through the point with x-coordinate −8.


for part a the values of t are 0.8 and 4 which are correct but for part b) I used the v=u + at formula where u = 12, a = -5 and t = 4

I get two answers which are 8 and -8 for both the values of t, however in the solutions they have used the v^2 = u^2 +2as formula, and got 14.9... , why wont the v = u + at formula work?

I had to look over this a couple of times to spot what was happening. Point A has x coordinate 8 but in question b they're asking about when the x-coordinate is -8 so you wouldn't be able to use your answer to part a.