已知向量\(\overrightarrow{a}=(-1,2),\overrightarrow{b}=(3,4)\).
\((I)\)若\(\left(2 \overset{⇀}{a}- \overset{⇀}{b}\right)/\!/\left( \overset{⇀}{a}+k \overset{⇀}{b}\right) \),求实数\(k\)的值;
\((II)\)若向量\(\lambda \overrightarrow{a}\)在\(\overrightarrow{b}\)方向上的投影为\(1\),求实数\(\lambda \)的值.