You have a rhombus ABCD. AB+BC+CD+DA = 80. CD = 2x+6. Since all sides of a rhombus are equal, you can set up the equation: 4(2x+6)=80 Distribute: 8x + 24 = 80 Subtract 24 8x = 80 - 24 Divide by eight: x = \frac {80-24}{8} Simplify: x = \frac{56}{8} x = 7 Let's check it by plugging it into th...