Determine the probabilities for X ≥ 81 assuming they are normally distributed with µ = 100 and σ2 = 36:

Mmm, let me see if I can remember this stuff.

sigma=6

so 81 is more than 3 standard deviations below the mean.

so p(X>=81)>99.7%

If you need more precision you can work out the z score 

Z=(81-100)/6=-3.1666666

then look up a z score table to find the associated area.  The tables are slighly diff and I don't know which version you are using.   I used this one.

http://designarchitectures.info/interior/z-score-table-negative-values

P(x>=81)=0.9987   or   99.87%