Here we use
r=ut+21at 2+r0
According to the text
u=i+2j
and part a) gave
a=0.5i−j
We assume the starting point is the origin so that r0=0.
At t=10+40=50 we obtain
r=(i+2j)50+21(a=0.5i−j)50 2
The distance is the magnitude of this vector