Find the height of the parallelogram.

100 - 36 = 64
square root (64) = 8

answer
height of the parallelogram = 8

So the missing length of the triangle is the height
In order to find the missing length of the triangle, use pythagorean theorem
a^2+b^2=c^2
6^2+X^2=10^2
36+x^=100
100-36=64
square root of 64 is 8
the missing height is 8
hope this helps!!