Ineed to rewrite the expression using addition, find the answer, then simplify but i don't know how. can someone explain this to me?

I'll just do 2 of them.

Remember that a positive minus a negative number is basically the positive plus positive. Example: 1-(-1) is equal to 1+1. The same goes for a positive plus a negative. Example: 1+(-1) = 1-1

For #4, we see 7-11. Using the rule, we can see that we can change this to    7+(-11).

For #11, the equation is -34-(-15)-(-18) so we can change this to:

-34+15+18. Always use the rule above!

The side x is the hypotenuse of the triangle.

And we know that the leg that is labeled with a line is also 6.

Use the Pythagorean theorem and solve for x.

a² + b² = x²
3² + 6² = x²
9 + 36 = x²
45 = x²
√45 = x
√9×5 = x
√9√5 = x
3√5 = x

The answer is 3√5

Hope this helps :)