Insert 50 for y and solve for x.
5x + 10 (50) = 800
5x + 500 = 800
5x = 800 - 500
5x = 300
x = 300 ÷ 5
x = 60

I divided by 100 and shifted the decimal point two places left.
An arithmetic sequence is defined by the property that the difference between each pair of adjacent terms is consistently the same. When this requirement is met, that constant is recognized as the common difference.
This particular sequence is arithmetic, as the difference between each term and the preceding one is
Thus, the common difference is 3.4, considering that the adjacent terms differ by 3.4