а) $$\vec{v} = 3\vec{a} - 3\vec{b} = 3(2; -5) - 3(-5; 2) = (6; -15) - (-15; 6) = (6 + 15; -15 - 6) = (21; -21)$$;
б) $$\vec{v} = 2\vec{a} - 3\vec{b} + 4\vec{c} = 2(4; 1) - 3(1; 2) + 4(2; 7) = (8; 2) - (3; 6) + (8; 28) = (8 - 3 + 8; 2 - 6 + 28) = (13; 24)$$;
в) $$\vec{v} = 3\vec{a} - 2\vec{b} - \vec{c} = 3(-7; -1) - 2(-1; 7) - (4; -6) = (-21; -3) - (-2; 14) - (4; -6) = (-21 + 2 - 4; -3 - 14 + 6) = (-23; -11)$$.
Ответ: а) $$\vec{v} = (21; -21)$$, б) $$\vec{v} = (13; 24)$$, в) $$\vec{v} = (-23; -11)$$.