First, subtract #color(red)(3/2)# and #color(blue)(1/4a)# from each side of the equation to isolate the #a# term while keeping the equation balanced:
#3/2 + 3/4a - color(red)(3/2) - color(blue)(1/4a) = 1/4a - 1/2 - color(red)(3/2) - color(blue)(1/4a)#
#3/2 - color(red)(3/2) + 3/4a - color(blue)(1/4a) = 1/4a - color(blue)(1/4a) - 1/2 - color(red)(3/2)#
#0 + (3/4 - color(blue)(1/4))a = 0 - 1/2 - color(red)(3/2)#
#2/4a = -4/2#
Now, multiply each side of the equation by #color(red)(4)/color(blue)(2)# to solve for #a# while keeping the equation balanced:
#color(red)(4)/color(blue)(2) xx 2/4a = color(red)(4)/color(blue)(2) xx -4/2#
#8/8a = -16/4#
#1a = -4#
#a = -4#