Find in what time a sum of money trebles itself at 10% per annum compound interest

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).

Asap! i'm not good math and i'm lost.

The perimeter of a rectangle is 42 inches. if the width of the rectangle is 6 inches, what is the length