58=Mean - 2SD
98= mean +3SD

from the 2 equations you get:

174=3 mean - 6SD
196= 2Mean + 6SD

Adding the equations:

370= 5 Mean
Mean = 74

Here's another way:

We know that there are 5 standard deviations between 58 and 98, a difference of 40. Therefore, one standard deviation is 40/5 = 8.

Now we can just add:

58 is two standard deviations below, so 58+8+8 = 74.

Or:

98 is three standard deviations above the mean, so 98-8-8-8 = 74.

Hope this helps!