This had me thinking for about 3-5 minutes!!! Try it for a few minute.

     Arrange the numbers 1 to 9 in the circles in such a way that each straight line of three(3) numbers sum up to 18, in the diagram below.

 Meaning:

• a+e+h = 18,
• b+f+i = 18,
• b+e+g = 18,
• c+f+h = 18 &
• d+e+f = 18.

We're given c & g to be 1 & 6 respectively (c = 1 & g = 6).

Here's the initial given diagram:

Though its my 1st official post but I'll post the answer after an improved number of vote likely better than the former.. Or Luckily someone might drill this!