This number is in complex form. To simplify a number in complex form, you simplify the two parts (real and imaginary) separately. For the given number 18- √-25, this is how we do it:
The real part "18" is already simplified, so we only have to deal with the imaginary part "√-25".
18 - √-25
To solve for the square root of -25, we factor it out first before simplifying:
18 -√(25)(-1)
18 - 5√(-1)
For imaginary numbers, √-1 = i. Using that relation, we can simplify it to:
18 - 5i, which matches with the answer in letter B.