First, multiply each side of the equation by the lowest common denominator (in this case #color(red)(8)color(blue)(a)#)to eliminate the fractions and keep the equation balanced:

#6/color(blue)(a) xx color(red)(8)color(blue)(a) = 3/color(red)(8) xx color(red)(8)color(blue)(a)#

#6/cancel(color(blue)(a)) xx color(red)(8)cancel(color(blue)(a)) = 3/cancel(color(red)(8)) xx cancel(color(red)(8))color(blue)(a)#

#6 xx color(red)(8) = 3color(blue)(a)#

#48 = 3color(blue)(a)#

We can now divide each side of the equation by #color(red)(3)# to solve for #a# and keep the equation balanced:

#48/color(red)(3) = (3color(blue)(a))/color(red)(3)#

#16 = (color(red)(cancel(color(black)(3)))color(blue)(a))/cancel(color(red)(3))#

#16 = a#

or

#a = 16#