Answer:
Dorothy = 13 years old
Step by step answer
Dorothy: x+7
William: x
x * (x+7) = 78
x^2 + 7x = 78
x^2 + 7x -78 = 0
What adds to 7, multiply to 78?
-6, 13
y=(x-6)(x+13)
Find the zeros
x-6=0
x=6 ( william is 6 years old)
x+13=0
x=-13 (age can’t be negative)
Dorothy = x+7
6+7 = 13