The question asks for an expression, not for a solution to an equation, so we're looking for an algebraic way of expressing half the sum of two numbers (which incidentally is the same as the average of two numbers). If we have two "ordinary" numbers, for example 11 and 7, we can easily add them together, giving us 18, then halve this sum (divide by 2) to give us 9. Note that we must add the numbers together first before halving. Without doing any arithmetic we could write this expression as (11+7)/2, the brackets telling us to carry out what's in the brackets first then divide by 2. But our question doesn't have two ordinary numbers, it's got one ordinary number and a symbol representing a number. In my example we used the ordinary number 11, but the question asks for any number, represented by x. Therefore, let's replace 11 by writing x so the expression we want becomes the answer (x+7)/2. Now we can see that algebra has given us a way of expressing a quantity for any value of x we care to replace it with - that's what algebra's all about!