Exercise algebra.equations.linear.balance

Description
Solve a linear equation using only balance rules.

Derivation

-21+6*x == 3*x

⇒ algebra.equations.linear.balance.var-left-minus, term=3*x

-21+3*x == 0

⇒ algebra.equations.linear.balance.con-right-plus, term=21

3*x == 21

⇒ algebra.equations.linear.balance.scale-to-one, factor=3

x == 7