Since the square root of a negative number is not a real number, then a negative radical will result in no real solution.
18.
-2(-9)+3
18+3
21
Explanation: -2(-9) When multiplying numbers that have both a negative sign, the answer is positive. Multiply normally.
19.
-6(-4)0
24(0)
0
Explanation: -6(-4) As I explained before, the answer would be positive.
24(0) Every number multiplied by 0 is 0.
20.
56+(-7)
49
Explanation: 56+(-7) Any problem like this is just basically going to be 56-7.
21.
-36+(-4)
-40
Explanation: -36+(-4) You are simply adding normally then adding the sign. This will only work if the signs are the same. (Signs are positive and negative.)
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45 cookies will serve 15 students.
You would multiply 15 by 2 that would give you 30, therefore you multiply 45 by 2 which will be 90.
90 cookies will serve 30 students
Your proportion should look something like this

After cross multiplication, you should end up with this equation

After dividing 4 on both sides, you should get that x=9