pippa's response is good but what you need to learn is to identify the information they've given you.
If something starts "from stationary", does that tell you something about its initial velocity? Of course it does. A stationary object isn't moving, so the magnitude of its velocity is zero.
u = 0
"and accelerates at a constant rate..."
It's basically just giving you the acceleration here.
a = 6
"The lorry passes the traffic light just as they go green"
It's telling you that they're starting from the same instant, because they'd be violating traffic laws to start from an earlier point. It also means that you need to use simultaneous equations.
It is travelling at a constant speed, meaning it's not accelerating (Newton's First Law)
u = 18
a = 0
The time and displacement of both will be the same when they meet (by definition, the point of overtaking is the time where the two vehicles meet ie have equal displacements).
The two unknowns are displacement (s) and time (t) using the equations pippa's given you. The equations used can be figured out by knowing what variables you have (u, a, s, t) and looking at the formulae sheet.
If you do them simultaneously by substitution, it cancels the displacement variable and gives you something to solve for time.