You can use pythagorean theorem and square 1.5 and 6 and those added together is the hypotenuse (the slope) squared. the answer is approximately 6.2
The correct answer is the second choice.
Paolo made an error when he cross multiplied the proportion. He should have multiplied the 3 and the 8 and also the 12 and the x.
The second line should be:
12x = 24