Answer: a = -1 (fourth choice)
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Work Shown:
q = (-4, 1) is one vector
r = (a,3) is another vector
The resultant vector is
q+r = (-4,1)+(a,3)
q+r = (-4+a,1+3)
q+r = (-4+a,4)
Multiply both sides by 7
7(q+r) = 7*(-4+a,4)
7(q+r) = (7*(-4+a),7*4)
7(q+r) = (-28+7a, 28)
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Since 7(q+r) = (-35, 28), we know that,
(-28+7a, 28) = (-35, 28)
which leads to
-28 + 7a = -35
when we equate the x components of each vector. Let's solve for 'a'
-28 + 7a = -35
7a = -35+28
7a = -7
a = -7/7
a = -1
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Check:
q = (-4,1)
r = (a,3) = (-1,3)
q+r = (-4,1)+(-1,3)
q+r = (-4+(-1), 1+3)
q+r = (-5, 4)
7*(q+r) = 7*(-5, 4)
7*(q+r) = (7*(-5), 7*4)
7*(q+r) = (-35, 28)
The answer is confirmed.