In a netball match between Class 3 and Class 4, 48 goals were scored. Class 4 scored 3 times as many goals as Class 3.
18. Class 3 scored goals
19. Class 4 scored ... goals

So we know 48 are scored collectively between the two teams, let’s call team 3 “x” and let’s call 4 “y” just so it’s easier to visualise:
We are told that x+y=48
And also told y=3x
If we substitute that in we get
X+3x=48
Collect like terms to get
4x=48 then divide by 4 to get
X=12
We now know that team 3 scored 12 goals
And if team 4 scored 3 times as many we can do 3*12=36 so team 4 scored 36
To make sure it’s correct 36+12=48
Hope this helped :)