How many possible solutions could f(x) = 6x^3 + 8x^2 - 7x -3 have?

We just started the introduction into circles and i have no idea how to do this.

Find two numbers if their sum is 91 and the ratio is 6: 7?

Aclassmate thinks that solving a system by graphing gives an exact answer when the lines appear to cross at a grid point, but only an approximate answer when they don't. explain why this isn't true.