Atheater has 10 seats in the first row and 30 seats in the 6th row. how many seats are in the 11th row?
(it's ok if you give me the equation, that's all i need and you)

Find the difference per row:

10 seats in the first row

30 seats in the sixth row:

30 -10 = 20 seats difference.

6-1 = 5 rows difference.

20 seats /  5 rows = 4 seats per row.

This means for every additional row, there are 4 more seats per row.

The equation would be:

Sn = S +(n-1)*d

Where d is the difference = 4

S = number of seats from starting row = 10

n = the number of rows wanted

S(11) = 10 + (11-1)*4

S(11) = 10 + 10*4

S(11) = 10 + 40

S(11) = 50

Check:

Row 6 = 30 seats

Row 7 = 30 + 4 = 34 seats

Row 8 = 34 + 4 = 38 seats

Row 9 = 38 + 4 = 42 seats

Row 10 = 42 + 4 = 46 seats

Row 11 = 46 + 4 = 50 seats.