The two expressions are equivalent because the associative property of multiplication is applied, which means that no matter where you put the parenthesis, the two expressions will still be equal.
15x-10=20
+10 +10
15x=30
\15 /15
x=2
you add 10 to both -10 (they cancek eachother out) and 20 (equals 30)
Then you divide 30 by 15 (equals 2) and 15 by 15 (they cancel eachother out)
Then you get x=2
Answer x = 6x3 there your welcome