This question is incomplete. I assume that you are looking for the number of adults and students.
Let a = adult
Let s = student
6a + 4s = 1000...A
a + s = 200...B
We have a system of linear equations in two variables, the variables a and s.
You can select any variable of choice to represent the adults and students.
Solve equation B for a or s (your choice).
a + s = 200
Solve for a or s (your choice).
s = 200 - a
Plug into equation A to find the value of a.
6a + 4(200 - a) = 1000
6a + 800 - 4a = 1000
6a - 4a = 1000 - 800
2a = 200
a = 200/2
a = 100
There are 100 adults.
Plug a = 100 into EITHER equation A or B to find students.
a + s = 200
100 + s = 200
s = 200 - 100
s = 100
There are also 100 students.
Did you follow?