MATLAB: How to find x for a given value of y

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I'm stuck on this, and no one seems to understand the question. I've defined several variables and made x an array of values. I then made an anonymous function and used fplot to graph it's outputs(F in this case) for every x value. Now, I just need to find the x value that give me F=90. The only way I can think to do it is to solve for x by hand and then type that into matlab but there has to be a simpler way. Any body know how to do this? Here's up to where I'm stuck:
 % Clear all windows and variables 
 clc 
 % Input the values for mass, height, friction coefficient, and gravity 
 m=18; % Mass (kilograms) 
 h=10; % Height (meters) 
 mu=0.55; % Friction coefficient (no units) 
 g=9.81; % Gravitational acceleration (m/s^2) 
 % Input a reasonable range for x 
 x=[-100:100]; 
 % Calculate F given the range of x 
 F=@(x) (mu.*m.*g.*(h.^2 + x.^2).^(1/2))/(x+mu.*h); 
 % Plot F as a function of x 
 fplot(F,[0,100]) 
 title('Force versus Distance') 
 xlabel('Distance (meters)') 
 ylabel('Force (newtons)') 
 grid on 
 % Find the x value that gives F=90 
 ??????????????

Best Answer

Add this code to the bottom of your code:
Fx = (mu.*m.*g.*(h.^2 + x.^2).^(1/2)) ./ (x+mu.*h);
% Plot the function.
figure;
plot(x, Fx, 'b-');
grid on
% Find the x value that gives F=90.
% This happens when the difference between Fx and 90 is a minimum.
[difference, index_At_F_Equals_90] = min(abs(Fx-90))
% Get the x value at that index.  Print to command window.
x90 = x(index_At_F_Equals_90)
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