Find an equation of the straight line tangent, at the given point, to the level curve of the given function passing through that point.
f(x, y) = x^{2}- y^{2} at (2, -1)

Sophia wrote an equation to represent the revenue of a movie theater for one day. she let x represent the number of child tickets sold and y represents the nunver of adult tickets sold. if a child ticket cost $4, an adult ticket cost $6, and the total revenue for the daybwas $420, which equation could sophia use to represent the number of chikd and adult tickets that were sold?