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1. 7 b - 3 b = 1 2
=> 4 b = 1 2
=> 4 b / 4 = 1 2 / 4
=> b = 3
2. z + 5 = 3 ( z + 5 )
=> z + 5 - 5 = 3 ( 5 ) + 3 z - 5
=> z = 1 5 + 3 z - 5
=> z = 1 0 + 3 z
=> z - z = 1 0 + 3 z - z
=> 0 = 1 0 + 2 z
=> 0 - 1 0 = 1 0 + 2 z - 1 0
=> - 1 0 = 2 z
=> - 1 0 / 2 = 2 z / 2
=> - 5 = z
3. a = 4 b
a + 4 b = 4 5
=> a + a = 4 5
=> 2 a = 4 5
=> 2 a / 2 = 4 5 / 2
=> a = 2 2 . 5
4. a = even numbers
a + ( a + 2 ) = 4 6
=> 2 a + 2 = 4 6
=> 2 a + 2 - 2 = 4 6 - 2
=> 2 a = 4 4
=> 2 a / 2 = 4 4 / 2
=> a = 2 2
=> a + 2 = 2 4
the values of these two numbers:
22 , 24
Hope I Can Help You^<>^

here is the steps :
7b - 3b = 12
4b = 12
b = 12/4
b = 3