w is being multiplied by 0.35. You want w alone, so divide both sides by 0.25.
0.35w/0.35 = -7/0.35
On the left side, 0.35/0.35 = 1, so all you have left is w.
On the right side, negative divided by positive is negative, and 7 divided by 0.35 is 20, so the answer is w = -20.
w = -20
Answer: w = -20