Half life question in matlab

Question: Metronidazole (used to treat infections) has a half life of H = 7.9 hours. Use solve to calculate [5 pt] the amount of time it takes for 50mg to decay to 20mg. Note that the decay constant is −ln(2)/H.

Code: solve(x*(-log(2))/(log(50/20)));

Getting the wrong answer, not sure how to solve it correctly.

[Image Analyst]

Didn't Star Strider and Roger already answer/help with this in http://www.mathworks.com/matlabcentral/answers/173236#answer_166376?