Review: a "solution" to an equation is a numerical value (or set of values) that makes the equation true.
Here you are given the linear equation 3x - 3y = 21 and the point (12,-5). If x=12 with y= -5, then (12,5) is a solution of 3x - 3y = 21.
Substituting, 3(12) - 3(-5) = 21. Is this true or false?
36 + 15 = 21 is obviously not true. Thus, (12, -5) is NOT a solution of 3x - 3y = 21.