If f(x) = x^2 + 5x + k and f(2) = 6, then f(4) = 12 26 28 36 38 The solution (here http://www.greenlighttestprep.com/module/gre-algebra-and-equation-solving/video/1013 ) finds the value of k and then figures out f(4). I didn't do that. Here's what I did. 2^2 = 4 and 4^2 = 16. 16-4 = 12, so I know that f(4) will be 12 more than f(2) Also, 5(2) = 10 and 5(4) = 20. 20 - 10 = 10, so I know that f(4) will be 10 more than f(2) In both functions we're adding k both times, so there's no difference there. So 12 + 10 = 22, so f(4) will be a total of 22 more than f(2) f(2) = 6, so f(4) = 6+22 = 28 (answer c) Is this a valid solution or did it only work because the numbers worked out nicely?