What equation can be written from this sequence? -50, -33, -16, 1

-50 + 17 = - 33
-33 + 17 = -16
- 16 +17 =1 

This is arithmetic sequence, where 1st term is -50,
and common difference is 17.
Formula for the term of arithmetic sequence 
a(n) = a(1) +d(n-1) = -50 +17(n-1) = -50 +17n - 17 = -67 +17n

So, we can write a formula
a(n) = -67 +17n