This is a system of two equations. You have some amount of money, let's call it X, that is invested at 5% interest, and some other amount of money, let's call it Y, that is invested at 4% interest. You know that together X and Y sum to a $28,000 total investment, and that their gained interest sums to a total of $1,260. So we can write this down mathematically as:
X + Y = 28,000
.05X + .04Y = 1,260
From here you can use your preferred method of solving a system of two equations (substitution, elimination, or row-reducing a matrix would be some options). You end up with the somewhat surprisingly simple answer of X = 14,000 and Y = 14,000.