Answer:
dy/dx at x=6 is 0.334
Step-by-step explanation:
The center difference method requires that the values of the function are given in equal intervals which is the case, and allows one to find the value for x = 6 using those of the function for x = 5.5 and for x = 6.5 as follows:

Create formulas!
x=number of adult tickets
y=number of student tickets
x+y=644 <em>total sold</em>
56+y=x <em>student was 56 less than adult</em>
Perform substitution
(56+y)+y=644
56+2y=644
2y=588
y=294
Substitute y into the formula we created
x+294=644
x=350
350 adult tickets were sold.