Make g the subject of the formula w = 7 - √g

Isolate the g

w = 7 - √g

Do the opposite of PEMDAS, first subtract 7 from both sides

w (-7) = -√g + 7 (-7)

w - 7 = -√g

multiply -1 (as -√g is the same as -1√g) to both sides

-1(w - 7) = √g
-1w + 7 = √g

to get rid of the square root, you must square both sides
Note: -1w is the same as -w
Note: you are squaring all the terms on the other side, not just one.

(-w + 7)² = (√g)²
g = (-w + 7)²
g = (-w + 7)(-w + 7) (note, this can be the answer your teacher wants, or                                            g = (-w + 7)²    )

Use the FOIL method (First, Outside, Inside, Last)

(-w)(-w) = w²
(-w)(7) = -7w
(7)(-w) = -7w
(7)(7) = 49

g = w² - 7w - 7w + 49

simplify (combine all like terms)

g =w² - 7w - 7w + 49

g = w² - 14w + 49

g = w² - 14w + 49 is your answer

hope this helps