Firstly, the order of operations states that we need to figure out everything in the parentheses first. 121 happens to be a square number (11²), so √121 equals 11.
42 - 11 = 31. So now, the problem simplifies to √16 × 31.
16 happens to square number too (4²), so √16 equals 4.
Our problem is now just 4 × 31, which equals 124.
You may think there's another answer too, as √121 is actually 11 OR -11, and √16 is actually 4 OR -4. But a negative number multiplied by another negative number is a positive number, so the answer would still be the same.
Hope this helps!