4x + 3y = 54 (1)
3x + 9y = 108 (2)
Multiply (1) by (-3)
-12x - 9y = -162
3x + 9y = 108
---------------------add
-9x = -54
x = 6
plug x = 6 into (1) to find y
4(6) + 3y = 54
24 + 3y = 54
3y = 30
y = 10
Answer
(6 , 10)
Hope it helps.
If 105 customers were the total, and 35% prefers cake, we must calculate 35% of 105, then we must do 105 multiplied by 35%, we can doit transforming the 35% in the fraction notation:

And the multiplication

Therefore, if we round it to the nearest whole number, the number of customers that prefer cake is 37.
37 customers prefer cake.