Please help me!

I'm assuming that we have

y = √[x^2 + 8x ]       square both sides

y^2 =  x^2 + 8x        complete the square on x

y^2 =     x^2 + 8x + 16  - 16   factor

y^2 = ( x + 4)^2  - 16      add 16 to both sides

y^2 + 16 =  (x + 4)^2      take the positive root

√[ y^2 + 16 ] = x + 4       subtract 4 from both sides

√[ y^2 + 16] - 4 = x       swap x and y

√ [ x^2 + 16 ] - 4 = y   = the inverse    [ where x is also > 0 ]