Answer: So, when x = 4, the value of the function y is 27.
Explanation: To find the value of the function y = 3x^2 - 21 when x = 4, you simply substitute 4 for x in the equation and calculate the result:
y = 3x^2 - 21
y = 3(4^2) - 21
y = 3(16) - 21
y = 48 - 21
y = 27
So, when x = 4, the value of the function y is 27.