First, subtract #color(red)(7.02)# from each side of the equation to isolate the #a# term and keep the equation balanced:
#-a/8.06 + 7.02 - color(red)(7.02) = 18.4 - color(red)(7.02)#
#-a/8.06 + 0 = 11.38#
#-a/8.06 = 11.38#
Now we can solve for #a# by multiplying each side of the equation by #color(blue)(-8.06)# which will also keep the equation balanced:
#-a/8.06 xx color(blue)(-8.06) = 11.38 xx color(blue)(-8.06)#
#a/color(blue)(cancel(color(black)((-8.06)))) xx cancel(color(blue)(-8.06)) = -91.7228#
#a = -91.7228#