I just need some help finding the equations to this problem and i will know how to do the rest.

Kenisha sells athletic shoes part-time at a department store. She can earn either $500 per month plus a 4% commission on her total sales, or $400 per month plus a 5% commission on total sales.

19) Write a system of equations to represent the situation.

there are two salary plans, a and b:

a = 500 + .04s
b = 400 + .05s