No sign change on TI-83 plus?

I'm using my TI-83 Plus to solve chemistry problems in which I'm trying to find the molarity of the H+ or OH- ion in a given acid or base. To do this, I graph 2 equations - however, when I solve for the intersection of these two equations, the calculator displays "NO SIGN CHNG". For example, I... show more I'm using my TI-83 Plus to solve chemistry problems in which I'm trying to find the molarity of the H+ or OH- ion in a given acid or base. To do this, I graph 2 equations - however, when I solve for the intersection of these two equations, the calculator displays "NO SIGN CHNG". For example, I enter 5.6 * 10^-10 for the first equation and (X^2)/(0.15-X) for the second, and can never get an answer.

However, I've tried doing the same on a TI-84 and it worked fine...what's wrong with my TI-83?
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