Let the commission sales = X
Salary Plan 1
salary per week = $400
commission sales = 2%

Salary Plan 2
salary per week = $300
commission sales = 18%

Becky's salary for both plans (inclusive of commission sales) would be:
Plan 1:

Plan 2:

In order to find how much Becky needs to make in sales for salary to be the same from both plans would be:

Therefore, the commission sales should be $625