Angles of the first line are the same of the second line because the lines are parallel and intersected by the same line
then
then
Now the opposite angles by the vertex like X and angle2 have the same value
then
and
the adjacent angle of each angle make a straight angle then the sum is 180
now if we sum x and angle 1 we have 180
we replace the value of x and solve for angle 1
opposite angle by the vertex of angle1 is angle3, then
and angle3 and 7 are the same