We can let x represent the number of hours she spent mowing lawns and y represent the number of hours she spent as a clerk. Then we can write the following equation to represent the total number of hours she worked:
x + y =35
We can also write an equation to represent the total amount of money she earned using the rates and the variables that represent the hours worked:
8.50x + 10y = ?
You would need to know the total amount of money she earned from both jobs to complete this equation. Then you can solve two equations with two unknowns using substitution.