differentiation equation

[lovely_nancy]

The function f(x) ax^3 + bx + 4 where a and b are constants, goes through the point (2,14) with gradient 21

Using the fact that (2,14) lies on the curve, find an equation involving a and b.


Could someone please help me with how to go on about this question please
im not sure here what method im using to find the equation

 

[galactus]

Since (2,14) is on the curve, we have

\(\displaystyle a(2)^{3}+2b+4=14\)

The derivative, or slope, at x=2 is 21. So, we also have:

\(\displaystyle 3a(2)^{2}+b=21\)

There are two equations with two unknowns. Solve for a and b.