Answer: b+158
We simply add the two subtotals to get the grand total. If we knew the value of b, then we'd be able to find an actual numeric result.
For example, let's say b = 100 pies were baked last year. That would mean b+158 = 100+158 = 258 pies were made over the two year period. However, since we don't know what b is, we just leave b+158 as is.