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Step 1

**open squares of all equations**

step 2

**add all equations**

u will have

x^2+y^2+z^2+2xy+2yz+2zx=49

as

**(x+y+z)^2=x^2+y^2+z^2+2xy+2yz+2zx**

now

in present case

(x+y+z)^2=49

**taking square root**

**x+y+z=+7 or ****x+y+z= -7**

step 2

u will have

x^2+y^2+z^2+2xy+2yz+2zx=49

as

now

in present case

(x+y+z)^2=49