To solve this problem you must apply the proccedure below:
1. You have:
A=3+12=15
B=4x
C=12
D=3x+1
2. Therefore, you have:
A/B=C/D
3. When you susbtitute the values of A,B,C and D, you obtain:
15/4x=12/(3x+1)
4. Now, you must clear the "x" to calculate its value, as below:
15(3x+1)=12(4x)
45x+15=48x
15=48x-45x
15=3x
15/3=x
5. Therefore, the result is:
x=5