3x^2+bx-5=0

I guess you want to solve the equation. Right?

3x^2+bx-5=0

For a general quadratic equation, ax^2+bx+c=0, there are two roots given by:

x=\frac {-b + \sqrt {b^2-4ac}}{2a} and x=\frac {-b - \sqrt {b^2-4ac}}{2a}

or, x=\frac {-b \pm \sqrt {b^2-4ac}}{2a}

In your case, a=3, b=b, c=-5.

Since b isn't known, I can't solve it further. If you aren't yet familiar with the quadratic equation, you'll have to supply a value for "b" so that the equation can be factored.