Final answer:
Using the property that opposite sides of a parallelogram are equal, we find that the length of AD is 21 units by setting up and solving the equation 3x + 15 = 21.
Step-by-step explanation:
To solve for AD in a parallelogram ABCD given that AD = 3x+15 and BC = 21, we can use the property that opposite sides of a parallelogram are equal in length. Therefore, AD and BC are congruent, which means their lengths are identical. With BC given as 21, we can set up the equation 3x + 15 = 21.
Solving for x:
- 3x + 15 = 21
- 3x = 21 - 15
- 3x = 6
- x = 2
Plugging the value of x back into the expression for AD:
- AD = 3(2) + 15
- AD = 6 + 15
- AD = 21
Thus, the length of AD is 21 units.