Answer
(a) The vectors should be parallel to achieve a resultant 7 $\mathrm{m}$ long (the unprimed case is shown below),
(b) anti-parallel (in opposite directions) to achieve a resultant 1 $\mathrm{m}$ long (primed case is shown),
(c) and perpendicular to achieve a resultant $\sqrt{3^{2}+4^{2}}=5 \mathrm{m}$ long (the double-primed case shown).
Work Step by Step
(a) The vectors should be parallel to achieve a resultant 7 $\mathrm{m}$ long (the unprimed case is shown below),
(b) anti-parallel (in opposite directions) to achieve a resultant 1 $\mathrm{m}$ long (primed case is shown),
(c) and perpendicular to achieve a resultant $\sqrt{3^{2}+4^{2}}=5 \mathrm{m}$ long (the double-primed case shown).
