Write:
x + 18 + 23 = 90
Next combine like terms (which is 18 + 23)
*Remember that combining like terms is adding or subtracting the numbers that are in one side of the equal sign (basically do not add 18, 23, and 90).
You will now get x + 41 = 90.
Now subtract 41 into 90
(90 - 41).
Your answer should be x = 49.
*Make sure this is the right answer by adding 49, 18, and 23 together. If it all equals to 90°, then the answer is correct.