Answer:
39
Step-by-step explanation
so d ÷ 3 = 13
first we want to change these numbers into a fraction so
d ÷ 3 =

13 is whole but we can still display it as 13÷1 =

line these up like this

now cross multiply the numbers that always line up diagonally so 13 and 3
13x3=39
next divide that answer by the number diagonal to the unknwon so 39÷1 = 39
that means d = 39