r = number of rows
r + 19 = number of seats in each row
r(r + 19) = 1170 ← total seats equals number of rows times number of seats in each row
r² + 19r = 1170 ← distributive
r² + 19r  1170 = 0 ← subtracted 1170
(r + 45)(r  26) = 0 ← factored
r + 45 = 0 or r  26 = 0 ← set to zero
r = 45 or r = 26 ← solved
r + 19 ← number of seats in each row
= 26 + 19 ← substituted
= 45 ← added
45 seats in each row
*McGenius*
