#### Answer

The price of the textbook is $165.

#### Work Step by Step

Let X represent the price of the calculator.
The textbook price is $45 more than the price of the calculator.
Thus, we obtain that the textbook price is 45 + X
Together, the calculator and the mathematics textbook cost 285 dollars.
So: X + 45 + X = 285
2X + 45 = 285
Subtract 45 from both sides:
2X = 240
Divide both sides by 2:
X = 120
Since the price of the textbook is 45 + X and X = 120, the price of the textbook is 45 + 120 = 165 dollars.