s1=-u1^2/2a1
s2=-u2^2/2a2
s2>s1+d
(If distance the car stops is bigger than the distance the lorry stops plus the initial distance then they will crash)
(sub s1 and s2 in)
-u2^2/2a2 > -u1^2/2a1+d
Switch 2a2 with whole left side of equation.
-u2^2/(-u1^2/2a1)+d > 2a2
Make the (d)a fraction by...