Let's first expand both sides of the equation:

Left side:
(х+3)(х+5)
= х^2 + 5х + 3х + 15
= х^2 + 8х + 15

Right side:
(3х+7)(х+3)
= 3х^2 + 9х + 7х + 21
= 3х^2 + 16х + 21

Now, set the two expanded expressions equal to each other:

х^2 + 8х + 15 = 3х^2 + 16х + 21

Next, move all terms to one side of the equation to set it equal to 0:

0 = 3х^2 + 16х + 21 - х^2 - 8х - 15
0 = 2х^2 + 8х + 6

Now, simplify the quadratic equation:

0 = 2х^2 + 8х + 6
0 = х^2 + 4х + 3

This is the final simplified form of the equation (х+3)(х+5)= (3х+7)(х+3).