Answer
($x + 658.6$) billions
Work Step by Step
Since $x$ represents the budget already in billions of dollars, we can take that as its unit of measuremnent, as in "$x$ billions". Since the budget for defense exceeded that of Education by 658.6 billions, we simply have to add the two, and give them the unit of billions:
Defense Budget: $$(x + 658.6) billions$$
