7.
已知函数\(f\left(x\right)=a{x}^{2}+1\left(a > 0\right),g\left(x\right)={x}^{3}+bx \).
\((1)\)若曲线\(y=f\left(x\right) \)与曲线\(y=g\left(x\right) \)在它们的交点\(\left(1,c\right) \)处具有公共切线,求\(a\),\(b\)的值;
\((2)\)当\(a=3\),\(b=-9\)时,若函数\(y=f\left(x\right)+g\left(x\right) \)在区间\(\left[k,2\right] \)上的最大值为\(28\),求\(k\)的取值范围.