If a line's slope is

, the slope of the perpendicular line is

in this case, the given line has a slope 4, which is 4/1, so the perpendicular line has a slope/gradient
The addition property of equality justifies this. You could also say that she simply subtracted 7, and then it would be the subtraction property of equality. This is the case because if you add or subtract (or multiply or divide) the same number on both sides of an equation, the equation will still be the same.
Hope it helps!
The answer is B because you multiply 14 days (2 weeks) to 5 which is 70 then you add the other 22cm and that's how you would end up with 92cm