"the sum of a number and eight is doubled, the result is one less than the number"
( x + 8 ) * 2 = -1 + x
Rewrite:
(x+8)*2 = x-1
2x + 16 = x-1 (distribute the 2)
x +16 = -1 (subtract x from both sides)
x = -17 (subtract 16 from both sides)
Checking (very important):
Is (-17+8)*2 = -17-1 ?
-18 = -18 ? yes.
