Answer
$x=3$
Work Step by Step
$4x+9=21$
First we move the 9 to the other side of the equation to isolate the "x" on one side. Moving the 9 will cause it to become negative.
$4x=21-9$
$4x=12$
Then we will divide the equation by 4 so we could get the value of the single "x"
$4x=12 /:4$
$\frac{4}{4}x=\frac{12}{4}$
$x=3$
