This situation can be interpreted as a linear function. In order to determine which is the equation of the line, you first calculate the slope of the function:
m = (y2 - y1)/(x2 - x1)
where (x1,y1) and (x2,y2) are a pair of points of the line. In this case you can relate these points to the information of the distance between superman and Lex, and the interval times. Thus, you have:
(x1,y1) = (11,1473)
(x2,y2) = (13,1359)
where x is the flight time of superman and y the distance between him and Lex.
By replacing in the formula for the slope m you obtain:
m = (1359 - 1473)/(13 - 11)
m = -114/2