Evaluate: 21(x+1)-6x=15x+21

Expression: $21\left( x+1 \right)-6x=15x+21$

Distribute $21$ through the parentheses

$21x+21-6x=15x+21$

Cancel equal terms on both sides of the equation

$21x-6x=15x$

Collect like terms

$15x=15x$

The statement is true for any value of $x$, because both sides are identical

$x \in ℝ$