the entire bottom is 21... but the rectangle part (top) is 11
so 21-11=10
10 is the total of the base for both triangles
which means the base for 1 triangle is 5
Then us Pythagorean therom which is A^2+B^2=C^2
But we're solving For B^2
so it's C^2-A^2=B^2
13^2=169
5^2=25
169-25=144
√(144)= 12
x=12
hope that makes sense