The difference of 8 and 3 is multiplied by one quarter of the sum of 7 and 5​

15

Step-by-step explanation:

the difference of 8 and 3 : 8-3 = 5

sum of 7 and 5 : 7+5 = 12

a quarter of that sum : 12/4 = 3

the difference from before is now multiplied by that quarter : 5 × 3 = 15

I know, you are a beginner, but you don't know basic arithmetic calculation of +, -, × , / ? really ?

yes because there is exactly one y value for every x value

for function there can't be more than one value for every x value