It's the sum of a geometric sequence.
Let's rewrite it a bit:

And now let's calculate this sum
:

Now let's calculate the initial sum:

It is 49. 7(9) -14 is 49.
8x+5= 69
Move +5 to the other. Sign changes from +5 to -5.
8x+5-5= 69-5
8x= 64
Divide by 8
8x/8= 64/8
x= 8
Answer : x= 8