Can anyone help please

Keith bought a soft drink for 4 dollars and 9 candy bars .he spent a total of 31 dollars .how much did each candy bar cost?

Find the gradient of f(x,y,z)equals=left parenthesis x squared plus y squared plus z squared right parenthesis superscript negative 1 divided by 2 baseline plus ln left parenthesis x right parenthesis x2+y2+z2−1/2+ln(xyz) at the point left parenthesis negative 2 comma 1 comma negative 2 right parenthesis(−2,1,−2).

Solve for n? [tex]z = \frac{3}{4} mn[/tex]