The wording of your question was confusing. I think you are trying to find the value of b that makes the stated parabola tangential to the x axis.
You need the curve to intersect the x axis and have a gradient of 0 at the point of intersection. Since you posted in the pre calculus part of the forum I assume you aren't able to use calculus to get a pair of simultaneous equations to solve.
Instead you'll have to use the properties of the graph. A parabola has only 1 turning point. If it is tangential to the x-axis then it cant cross it. taken together this means that the equation f(x) = 0 has only 1 solution, ie the graph has a double root.
If a parabola has a double root it must be possible to factorise it as follows:
expanding the brackets:
you know that the equation is
coefficients of (constant term)
in case you dont believe me, the graph is plotted here:
You can see it is tangential to the x axis as required.
NB: the above solution is not unique. b=-20 works too. You can get this by taking the negative square root instead of the positive one when comapring coefficients.