The altitude QT and PM of triangle PQR

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).

Jenisey has $24 to spend on seven pencils. after buying them she had $10. how much did each pencil cost.?

Having a hard time understanding this question (see attachment).