Exercise algebra.equations.linear.balance

Description
Solve a linear equation using only balance rules.

Derivation

-30 == -7*x-9

⇒ algebra.equations.linear.balance.con-left-plus, term=9

-21 == -7*x

⇒ algebra.equations.linear.balance.scale-to-one, factor=-7

3 == x

⇒ algebra.equations.linear.flip

x == 3