The sprintf function returns formatted string or a character array which may be assigned to a variable.
In the following code given,
n = 4;
Fuel = JP5;
sprintf('%s_xvalues_upper_%d', 'Fuel', n) = flame_x1([upper_flame_width]);
In the above code flame_x1 should be on the left side of equals so that it attains the value from sprintf function.
Refer to the sprintf documentation for more details: