Triangle

Area =  12sqrt (46) =  (1/2) (15) * altitude

12sqrt (46)   / 7.5  =  altitude ≈  10.85

By Heron's formula, the area of the triangle is 

$\sqrt{23*(23-11)*(23-15)*(23-20)}=\sqrt{23*12*8*3}=\sqrt{6624}=12\sqrt{46}$.

Because side * altitude / 2 = area, then.

$\frac{20x}{2}=12\sqrt{46}$

$10x=12\sqrt{46}$

$x=\frac{6\sqrt{46}}{5}$.