All you do is solve for z. So 8z-13=7. To solve for a variable you have to do the opposite to cancel out the numbers on each side. First you add 13 over to 7 to cancel out 13 on the z side. So 8z=20 is what you have left. Then you divide 8 on both sides to cancel out 8 on the z side. what you are left with is z=2.5. Lastly you plug and check. 8(2.5) - 13 = 7, 20-13=7. And thats correct so z is 2.5.
Your answer would be 4, because the equation for the circumference of a circle is π × diameter, so if you have 4 × π as the circumference, then 4 is the diameter.
I hope this helps!
Using the slope-intercept form, y=mx + b to find the slope I found that correct answer is 2/5.