⛔️ 15 points ⛔️ you sm to whoever answers this!

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