6/13 = 3/c
You need to cross multiply to solve this.
That means you need to multiply the numerator on the left by the denominator on the right and the denominator on the left by the numerator on the right.
6c = 39
c = 6.5
To check it, input the number you got into the c in the equation.
6/13 = 3/6.5
0.46 = 0.46