Solve 1 expression
do everything in great detail (well, just a very great detail)

Remember the distributive property
a(b+c)=ab+ac

so
first distribute
(2/3a-0.8)(a-0.8)=0.64
focus on the left side

(2/3a-0.8)(a-0.8)=
(a)(2/3a-0.8)+(-0.8)(2/3a-0.8)=
(2/3a^2)+(-0.8a)+(-1.6/3a)+(0.64)=
2/3a^2-4/3a+0.64
now we do
2/3a^2-4/3a+0.64=0.64
minus 0.64 both sides
2/3a^2-4/3a=0
factor
(a)(2/3a-4/3)=0
set each to zero
a=0

2/3a-4/3=0
add 4/3 both sides
2/3a=4/3
times 3 bohth sides
2a=4
divide 2
a=2

a=0 or 2