The straight lines are asking to find the modulus or the size of the complex number.
For a complex number z = x + iy, [z] = √(x^2 +y^2)
Where x = Real Number, y = Imaginary number.
For 3 +i, x = 3, y = 1 (Note y is what number that is attached to i)
[3 + i] = √(3^2 +1^2) = √10 = 3.162