Here we have right triangles, which we can solve using trigonometric reasons, bceause we know the acute angles and the hypothenuse of the bigger right triangle.
Let's the sin function
Then, we use the cosine function
Now, we use the sin to find in the middle size right triangle
Changing those words into an equation, we get: 7+3x = 19 Subtract both sides by 7 3x = 12 Divide both sides by 3 x = 4 That number is 4. Have a nice day! :)