Setup an equation to solve for the unknown score. The catch here is to first convert the percentage of 91 into a one sided z-score. Use the percentile to z-score calculator with one-sided area to get a z of 1.34.  So now we just plug and chug

(x-73)
---------   =  1.34
    12

x-73  =  16.08

x = 89.08

So Sandra need to get just a bit more than an 89 to beat 91% of her math class (go Sandra!).  You can also solve this visually by using the interactive graph of the standard normal curve and entering the mean of 73 and standard deviation of 12 into the one-tailed area graph.  Then just hover over the bell-curve until the shaded area is at 91%, which confirms our algebraic solution above.