binomial distribution, health and medicine homework help

I do not understand how to work the formula

Use the binomial distribution for this question. A doctor knows from experience that 10% of the patients to whom she gives a certain medication will experience undesirable side effects. Assume the doctor gives medication to the next 12 patients. Referring to the binomial distribution for the medication, find the probability that at most two of these patients will experience undesirable side effects. Give your answer to three decimals, e.g., .987

 
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