Answers

2016-01-31T18:00:12+08:00
In math, those letter you were talking about are called variables. 

1.) x+9=4\\x=4-9\\x=-5\\checking:\\-5+9=4\\4=4

2.) y-12=15\\y=15+12\\y=27\\checking:\\27-12=15\\15=15

3.) 7=3+z\\7-3=z\\4=z\\checking:\\7=3+4\\7=7

4.)  11=4-w\\11-4=-w\\(-1)(7=-w)(-1)\\-7=w\\checking:\\11=4-(-7)\\11=11

5.) z-8=12\\z=12+8\\z=20\\checking:\\20-8=12\\12=12

6.)  \frac{4x}{4}= \frac{28}{4}  \\x=7\\checking:\\4(7) = 28\\28=28

7.) 28-2x=3\\-2x=3-28\\-2x=-25\\ \frac{-2x}{-2} = \frac{-25}{-2} \\x=12.5\\checking:\\28-2(12.5)=3\\28-25=3\\3=3

8.) 4+3x=5\\3x=5-4\\ \frac{3x}{3} = \frac{1}{3} \\x=0.33\\
The answer is an irrational number.

9.)  8+2x=0\\2x=0-8\\ \frac{2x}{2} = \frac{-8}{2} \\x=-4\\checking:\\8+2(-4)=0\\8+(-8)=0\\0=0

10.)  -11x=121\\\frac{-11x}{-11} = \frac{121}{-11}\\x=-11\\checking:\\-11(-11)=121\\121=121

Know the inverse property of the property used then apply the property in order to find the value of the variables. For checking, substitute the variables with their corresponding values. 

Answers:
1.) x = -5
2.) y = 27
3.) z = 4
4.) w = -7
5.) x = 20
6.) x = 7
7.) x = 12.5
8.) x = 0.33
9.) x = -4
10.) x = -11
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I feel that I've made a typo so just in case, just look check the solutions. (I'm talking about the bold letters.)

I hope it help. ☺
look/check *
look at/check
Nevermind that. Just in case, better check the solutions I've put. ☺
thanks