Read the attached pic. i don't remember ever being taught

This is notation referring to the nCr combination formula. 

In this case, n = 7 and r = 3

Plug those values into the formula below and evaluate
n C r = (n!)/(r!*(n-r)!)
7 C 3 = (7!)/(3!*(7-3)!)
7 C 3 = (7!)/(3!*4!)
7 C 3 = (7*6*5*4!)/(3!*4!)
7 C 3 = (7*6*5)/(3!)
7 C 3 = (7*6*5)/(3*2*1)
7 C 3 = 210/6
7 C 3 = 35

Note: the exclamation marks refer to factorial notation.

Therefore, the answer is choice D) 35