Equation can be explained like this:

Geraldo has $40 already saved. He needs more money to equal $129 for CD. Lets say y is that additional amount of money that he needs. So he needs $40+y to equal 129. That y is, if he saves money for four months every week, y=16x.

So we have $40+y=$129

$40+16x=$129

To find out how much Geraldo should save in order to buy CD then equation need to be solved for x:

16x=129-40

16x=89

x=5.5625 = $5.7

So he needs to save 5.7$ each week which is 16*5.7=89$ totaly saved for four months which summed with $40 equals $129 for CD.