Matlab plotting and Mfile functions

can somebody please explain this question ... i don`t even understand what the question wants me to do. A mineral survey has found good surface concentrations of gold in a certain area of dimensions 10 km 10 km. The concentration in ppm (parts per million) is given by the function f (x; y) = 2 + 0.1x + 0:008(x + y)^2+sin(x-y) first it wants me to plot a surface which is easy but then this is the second part pf the question which i don`t understand.

The only track through the area lies along the line y = 5. Write a MATLAB program to plot the concentration g(x) = f (x; 5) on the track for 0 <= x <= 10, and, using a function M-file and the fzero command, find the interval for x in which the concentration g(x) is greater than or equal to 4.