What is the distance of line ac with coordinates (0,5) and (3,10.5)

You can use the distance formula:  d  =  √[ (x2 - x1)² + (y2 - y1)² ]

(x1, y1) = (0, 5)     --->     x1 = 0   and   y1 = 5

(x2, y2) = (3, 10.5)   --->   x2 = 3   and   y2 = 10.5

d  =  √[ (3 - 0)² + (10.5 - 5)² ]   =   √[ (3)² + (5.5)² ]   =  √[ 9 + 30.25 ]    =   √[ 39.25 ]