What is the value of a2 – b2 if a + b = 6 and a – b = –3?

How do you do this geometry problem

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).

Choose the more precise measurement. 26.4 cm or 8.39 cm