In the figure below

Explanation

In the figure, two corresponding angles have degree measures that are represented by the algebraic expressions 9x - 15^o and 6x + 30^o.  Since \(l_1 \parallel l_2 \), this pair of corresponding angles are congruent.

• Since the angles are congruent, their measures are equal
  • 9x - 15^o = 6x + 30^o
• To eliminate 6x from the right side, subtract 6x from both sides.
  • 3x -15^o = 30^o
• To isolate the variable term 3x, undo the subtraction of 15^o by adding 15^o to both sides: 30^o + 15^o = 45^o
  • 3x = 45^o
• To isolate x, undo the multiplication by 3 by dividing both sides by 3
  • x = 15^o