Simplify and express the result in the form (a+ib): (3+√-16)-(4-√-9)?

You need to learn the meanings of the symbols.

√-16 = √-1 x √16 = i4

√-9 = √-1 x √9 = i3

Adding spaces to improve readability and substituting:
(3 + √-16) - (4 - √-9) = (3 + i4) - (2 + i3) = 3 + i4 - 2 + i3 = 1 + i7 << ANSWER

There is a snake in this problem. That means you are going to get bit if you are not careful. "Square root" implies two values, one positive and one negative. For example, √-1 = ±i not just i. I have only used the positive values in this demonstration, but there are several other equally valid answers if combinations of + and - are used.

This becomes important when you work on functions, defined as a process that gives one output for every input. A square root is not a function because it gives two outputs for every input.