Find the value of a such that the points (3, a) and (5, 2a) lie on a line with slope m=1/3.

remember the formula for finding slope
Y2-Y1/X2-X1=m
so we are giving Y2=2a,Y1=a,X2=5,X1=3 and m=1/3
2a-a/5-3=1/3
a/2=1/3
by cross multiplying we have 3a=2
a=2/3