-23 is greater than or equal to -5u + 7

Subtract 7 from both sides of the inequality:
$-23 - 7 \ge -5u + 7 - 7$
$-30 \ge -5u$

Divide both sides by -5. Remember to reverse the inequality sign when dividing by a negative number:
$\frac{-30}{-5} \le \frac{-5u}{-5}$
$6 \le u$

The solution can also be written as:
$u \ge 6$