y varies directly with x means y = kx, where k is the constant value. To find x when y = -7 first we have to find k, using y = 35 and x = 5
y = kx
35 = k(5)
35/5 = k
k = 7
Find x when y = -7. Remember k is constant and it stays 7.
y = 7x
-7 = 7x
-7/7 = x
x = -1
Answer: x = -1 when y = -7