Answer:
For your first problem you want to see what all the sides would equal
You would have to combine them all into one big equation
5x-7+3x+1+4x=30 (now you want to combine like terms)
12x-7+1=30 (combined the x's together)
12x-6=30 (added -7 and 1)
12x=36 (added 6 to each side)
x=3 (divided 12 to each side)
Now you wanna plug 4 in for x for each side to find out the largest to smallest
AB=5(3)-7= 8
BC=3(3)+1= 10
AC=4(3)= 12
So in order from smallest to largest would be angle C, B, A because
Angle C is the smallest angle because the sides AC and BC are the longest so they form the smallest angle
Then it would be angle A being the largest because the smallest sides in length AB and BC make up the biggest angle
The next problem would be that in order for it to have to be a triangle the two smallest sides needs to add up higher than the third side so
11, 11, 24, 11+11=22, which is less than 24 so not this one
18,12,9, 9+12=21 which is greater that 18 so this one would be a triangle
9,10,19, 9+10=19 which 19 isn't greater than 19 so this one is not a triangle
4,7,23, 4+7=11 which is less than 23, so this one is also not a triangle
So your answer would be 18cm,12cm,9cm
. Substituting, 
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is 
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