For this web exercise, assume all angles are measured in degrees.
In particular:
Rays that:

Note:
If three points are on a line
with $\,B\,$ between $\,A\,$ and $\,C\,$,
then
$\,\overrightarrow{BA}\,$
and
$\,\overrightarrow{BC}\,$
are opposite rays.
Recall that both $\,A{}B{}C\,$ and $\,C{}B{}A\,$
are notation for ‘$\,B\,$ is between $\,A\,$ and $\,C\,$’.
Two angles are a linear pair
iff

Note: If $\,\angle 1\,$ and $\,\angle 2\,$ are a linear pair, then $\,m\angle 1+ m\angle 2 = 180^{\circ}\,$.
Two angles are vertical angles iff the sides of one angle are opposite rays to the sides of the other. 
Note: Vertical angles are the ‘opposite angles’ that are formed by two intersecting lines.
Note: If $\,\angle 1\,$ and $\,\angle 2\,$ are vertical angles, then $\,m\angle 1 = m\angle 2\ $.
Parallel lines are studied in more detail in a future section, Parallel Lines.
The symbol ‘$\,\parallel\,$’ is used to denote parallel lines.
The sentence
‘$\,\ell\parallel m\,$’
is read as
‘$\,\ell \,$ is parallel to $\,m\,$’,
and is true precisely when line
$\,\ell\,$ is parallel to
line $\,m\,$.
The symbol ‘$\,\perp\,$’ is used to denote perpendicular lines.
The sentence
‘$\,\ell\perp m\,$’
is read as
‘$\,\ell \,$ is perpendicular to $\,m\,$’,
and is true precisely when line
$\,\ell\,$ is perpendicular to
line $\,m\,$.
$7x15= 2x+55$  (vertical angles have equal measures) 
$5x15=55$  (subtract $\,2x\,$ from both sides) 
$5x=70$  (add $\,15\,$ to both sides) 
$x=14$  (divide both sides by $\,5\,$) 
On this exercise, you will not key in your answer. However, you can check to see if your answer is correct. 
PROBLEM TYPES:
