Don't have the enough rep to comment, so I'll just add it as an answer. I didn't find the answer at least I just tried to reason why the first answer could be correct. i.e. 11.
My thinking is based on lateral and pattern as the user said to look at the tags for hints. According to that we don't have a certain pattern to it exactly I just tried to see it differently. I don't consider it a pattern but just pairing up.
$28 + 32 = 35 \qquad [ 7\cdot4 \qquad + 7\cdot5-3 = 7\cdot5]$
$25 + 49 = 39 \qquad [ 7\cdot4-3 \; + 7\cdot7 \quad~~ = 7\cdot6-3]$
$21 + 46 = 42 \qquad [ 7\cdot3 \qquad + 7\cdot7-3 = 7\cdot6]$
$18 + 14 = X \qquad [ 7\cdot3-3 ~\, + 7\cdot2 \quad~~ = 7\cdot2-3]$
Taking pairings up and down along with the sideways we have 12 numbers so 6 pairings in all, mentioned below.
$7\cdot4~~$ & $~~7\cdot4-3~~$ (up and down)
$7\cdot5~~$ & $~~7\cdot5-3~~$ (sideways first row)
$7\cdot7~~$ & $~~7\cdot7-3~~$ (up and down)
$7\cdot6~~$ & $~~7\cdot6-3~~$ (up and down)
$7\cdot3~~$ & $~~7\cdot3-3~~$ (up and down) & the last pair
$7\cdot2~~$ & $~~7\cdot2-3~~$ (sideways) that's because the last number remaining was $7\cdot2$ (14 only) and could be paired up with $7\cdot2-3$ i.e. (11)
Since the answer 11 was deemed to be correct by the user, I'm not sure how the person who answered came up with it or why he came up with 11 only. There seems to be no indication as such where 11 could be concluded that way.
Note to the questioner: Since I didn't come up with the answer and it's just a mere speculation please don't mark it as correct. I didn't had the considered rep for commenting otherwise this would just have been split into comments. Thanks.