Let S = the amount of sales.
She earns 4% of her sales, this would be written as 0.4S ( you multiply the percent as a decimal by the amount of sales).
You then need to add that amount to her salary so you now have 0.4s + 2100
She wants to earn at least 2900 so the inequality becomes:
2900 ≥ 0.4s + 2100
Her sales would need to be:
800 ≥0.4s
s≥ 800/0.4
s ≥ 2000