Answer:
BD= 56units
Explanation:
Since, Diagnols of a parallelogram bisect each other.
Therefore in parallelogram ABCD where AC and BD are the diagnols wich intersect at Point E, we have, BE=DE
Therefore, x^2-21=4x
x^2-21-4x=0
x^2-4x-21=0
x^2-7x-3x-21=0 [middle term splitting]
x(x-7)+3(x-7)=0
(x+3)(x-7)=0
Hence, x= -3 and x= 7
Since x cannot be negative because no side can have negative values
Therefore, we have x=7
Now, BD= BE+DE [Since E is the intersecting point of BD]
= 4x+x^2-21
= 4(7)+7^2-21
=28+49-21= 56 units