In order to find the value of x, which coincides with one of the diagonals of the rhombus, use the following formula for the area of a rhombus:
A = (d1 · d2)/2
where d1 and d2 are the two diagonals of the figure.
In this case, you have:
A = 45
d1 = BD = 6
d2 = AC = x
replace the previous values of the parameters into the formula for A, and solve for x, as follow:
45 = (6·x)/2
45 = 3x divide by 3 both sides
45/3 = x
15 = x
x = 15
Hence, the value of x in the given rhombus ABCD, is 15 units