Hi there!
Let's start with finding the measurement of angle A. Looking at the image, we know that the larger, outer triangle is equilateral. Therefore, we know that the inner, smaller triangle is isosceles. So, to find the measure of angle A, we need to solve this here equation:
180 = 90 + 2A
90 = 2A
A = 45
Now that we know that angle A is equal to 45 degrees, we can take the total value of the angle of the equilateral triangle and subtract the value of angle A from it to find the value of angle B.
B = 60 - 45
B = 15 degrees
Hope this helps!! :)
If there's anything else that I can help you with, please let me know!