Can you please help me with this question

Point O is the intersection of 2 roads which cross at right angles; one road runs from north to south, the other from east to west. Car A is 100 m due west of O and travelling east at a speed of 20m/s, and Car B is 80 m due north of O and travelling south at 20 m/s.

a) show that after t seconds their distance apart, d metres is given by

d^2 = (100-20t)^2 + (80-20t)^2

b) show that this simpifies to

d^2 = 400((5-t)^2 + (4-t)^2 ).

c)show that the min distance apart of the 2 cars is 10 squareroot 2 m