Question 893779
Two points as given in question are (12a,5b) and (3a,5b)

length of a line segment is given by = sqrt( (( X1-X2) ^2) + ((Y1-Y2)^2))  )

Thus

= sqrt( (( 12a-3a) ^2) + ((5b-5b)^2))  )

= sqrt ((9a ^2) +0 )

length= 9a