Let x be the cost of a hamburger kid meal and y the cost of a chicken nugget kid meal
Monday: 6 hamburger kid meals and 7 chicken nugget kid meals for a total of $39:

Thursday: $42 on 8 hamburger kid meals and 6 chicken nugget meals:

System of equations:

To solve by elimination:
1- Multiply first equation by 4 and second equation by -3

2- Add both equations you get in step 1:

3- Use the equation you get in step 2 to solve y:

4- Use the value of y to solve x:

Solution:

Each hamburger meal costs $3 and each chicken nugget meal costs $3