For a certain exam, a score of 58 was 2 standard deviations below mean

These are concepts of normal distribution. You don't need to delve much into details but you must understand what 1 SD, 2 SD etc mean.

Say you have a distribution of weights in a class of children. The mean weight of the class is, say 46 kgs. If I tell you that 1 standard deviation is of 5 kgs, it means that 1 SD below mean is 5 kgs less than 46 kgs which is 41 kgs. This also means that 34% children have weight ranging from 46 kgs to 41 kgs but you don't have to worry about that.

So 2 SD below mean will be 2*5 = 10 kgs less than 46 i.e. 36 kgs.

Similarly, 3 SD above mean will be 3*5 = 15 kgs more than 46 i.e. 61 kgs.

In the original question, you are given that 58 is 2 SD below mean and 98 is 3 SD above mean. So from 58 to 98, we have 5 SDs.
\(\frac{{98 - 58}}{5} = 8\) = SD

Since SD is 8 so mean is at \(58 + 2*8 = 74\)