First you need to consider order of operations PEMDAS and distribute 7 to what is inside the parentheses. Now we multiply.
42=7(z-3)
42=7z-21
Now we need to isolate z, the variable of this equation. Add 21 to both sides to cancel it (-21+21=0) 42+21=63.
63=7z
Now we know that 7 of this variable equals 63. To get z by itself, divide by 7 on both sides. 63 divided by 7 is 9.
z=9
