Answer: The required value of x is 3.
Step-by-step explanation: We are given to find the value of x if the quadrilateral shown in the figure is a rhombus.
From the figure, we note that
the lengths of the three sides of the rhombus are 5x units, (4x + 3) units and 15 units.
We know that the lengths of all the four sides of a rhombus are equal, so we get

Thus, the required value of x is 3.