If you read the question carefully,
$78 000 is actually the total amount he invested from two separate investments, one at 7% Simple Interest and the other at 5% Simple Interest. So what can be done is basically let one of these investments be and unknown variable x so that the other would be represented by 78 000 - x, as per how I’ve done it here. From there you just apply the Simple Interest formula to each investment and equate the total (in terms of x) with the stated total of $4800, then just solve the equation to find x, which is what you’re looking for. Hope this is clear enough for you to understand. All the best! :)