Let call the number in the blank A, then:
-8(2x+6)+6(-3+Ax)=26x-66
If we solve the left side, using distributive property we get:
-16x - 48 - 18 + 6Ax = 26x - 66
-16x + 6Ax - 66 = 26x - 66
Adding 66 on both sides:
-16x + 6Ax - 66 + 66 = 26x - 66 + 66
-16x + 6Ax = 26x
It means that:
-16 + 6A = 26
Solving for A:
-16 + 6A ´+ 16 = 26 + 16
6A = 42
6A/6 = 42/6
A = 7
So, the answer is 7