Need solving these two vector problems.

see below

Step-by-step explanation:

Here, we'll use an ordered pair <a, b> to represent each vector's two components. The rules are ...

multiplying a vector by a scalar multiplies each component by that scalarmultiplying a vector by a scalar multiplies its magnitude by the magnitude of the scalarthe magnitude of a vector is the square root of the sum of the squares of its components1.

For A = <2.5, -3.5>, |A| = √(2.5²+(-3.5)²) = √18.5 ≈ 4.30

2A = <5, -7>; |2A| = 8.60-2A = <-5, 7>; |-2A| = 8.60A/2 = <1.25, -1.75>; |A/2| = 2.152.

A = |A|<cos(43.9°), sin(43.9°)>

B = |B|<cos(154.8°), sin(154.8°)>

C = <0, -25.8>

The sum being zero gives rise to 2 equations in 2 unknowns.

  |A|cos(43.9°) +|B|cos(154.8°) = 0

  |A|sin(43.9°) +|B|sin(154.8°) = 25.8

Using Cramer's rule to find the solution, we get ...

  |A| = 25.8cos(154.8°)/(cos(154.8°)sin(43.9°) -sin(154.8°)cos(43.9°))

  |A| = 25.8cos(154.8°)/sin(43.9° -154.8°)

  |A| ≈ 24.9887

  |B| = -25.8cos(43.9°)/sin(-110.9°)

  |B| ≈ 19.8995

Your cleary not see the answer is 25

Element 17 is chlorine it is a group 7 element(halogen) and belongs to period 3 of the periodic table