Answer:
x = 38
Step-by-step explanation:
Here, we can use the area formula A = 0.5(Bh) to solve for x.
In this case, A = 399, B = x, and h = 21
[tex]399 = \frac{21}{2} x\\x = \frac{2*399}{21} \\x = 38[/tex]
