6(z+2)-2(2z+5)=18 show step by step

Follow PEMDAS.

Note: 
Parenthesis, Exponents (and roots),  Multiplication,  Division,  Addition, Subtraction)

Because you are trying to isolate the z (variable), you cannot do parenthesis. Instead, distribute 6 to z and 2, and -2 to 2z and 5

6(z + 2) = 6z + 12
-2(2z + 5) = -4z - 10

6z + 12 - 4z - 10 = 18

Next, combine like terms. Combine terms with common variables together, as well as constants.

6z - 4z = 2z
12 - 10 = 2

2z + 2 = 18

Next, isolate the z, do the opposite of PEMDAS. Note that there is an equal sign. What you do to one side, you have to do to the other.

2z + 2 = 18

subtract 2 from both sdies

2z + 2 (-2) = 18 (-2)
2z = 18 - 2
2z = 16

isolate the z, divide 2 from both sides

2z = 16
2z/2 = 16/2
z = 16/2
z = 8

8 is your answer for z

hope this helps