Question 1021862
{{{ax^3+bx}}}


{{{a*1^3+b*1=3}}}
{{{a+b=3}}}


{{{df/dx=3ax^2+b=7}}}
specified, at x=1 is that slope.
{{{3a*1^2+b=7}}}
{{{3a+b=7}}}



System to solve for a and b is  {{{system(a+b=3,3a+b=7)}}};
and elimination of b will give you {{{a=2}}}.