Five more than eight times a number is the same as twenty-one less than a...

Let x be the number.
Eight times the number is $8x$. Five more than eight times the number is then $5+8x$.
Twenty-one less than the number is $x-21$.
Since five more than eight times the number is the same as twenty-one less than the number, then $5+8x=x-21$. To find x
$5+8x=x-21$
$5+7x=-21$
$7x=-26$
${\displaystyle x=-\frac{26}{7}}$
The number is then $-\frac{26}{7}$