The four marked angles are the interior angles of a
  quadrilateral. Hence 






 
  <p/>  
<math xmlns="http://www.w3.org/1998/Math/MathML">
<mrow><mi>x</mi><mo>=</mo><mn>360</mn><mo>-</mo><mo stretchy="false">(</mo><mn>105</mn><mo>+</mo><mn>115</mn><mo>+</mo><mn>125</mn><mo stretchy="false">)</mo><mo>=</mo><mn>15</mn></mrow></math>
  <br />
  <br />