Answer: 5.5 which is choice B
Have a look at the diagram I posted below. I marked on your image to add in another angle. This angle is also 20 degrees because of congruent alternate interior angles (horizontal lines are parallel). This angle I add in is the reference angle of the triangle
opposite the reference angle is the vertical side x
adjacent to the reference angle is the horizontal side 15
We'll use the tangent rule. Make sure your calculator is in degree mode.
tan(angle) = opposite/adjacent
tan(20) = x/15
15*tan(20) = x <<-- multiply both sides by 15
x = 15*tan(20)
x = 5.45955 <<--- use calculator; this is approximate
x = 5.5 <<--- round to one decimal place
