Exercise algebra.equations.linear.balance

Description
Solve a linear equation using only balance rules.

Derivation

-21 == -7*x

⇒ algebra.equations.linear.flip

-7*x == -21

⇒ algebra.equations.linear.balance.negate

7*x == 21

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

x == 3