Hello there

To write the give lpp into standard form a few things have to be done.Since x3 is unrestricted in sign it should be into a format where its not.A slack variable should be introduced in the second condition.So the standard form would be

Min z=2x1+7x2+r1-s1

St x1+r1=7

3x1+x2-s1=2

x31-x32>=0

For further clarification just comment below.You can clear your doubts by watching NPTEL videos of Linear Programming and Simplex Method on YouTube.

Hope I was able to help.