x=4
You can find this by looking at the relationship between the two triangles, the side of the larger triangle that is 15 units is 5 times the length of the same side of the smaller triangle. The side length of the larger triangle that is 25 is also 5 times the same side length of the smaller triangle. So we can look at the last remaining side and see that fives time that side which is 4, would equal 20. So we take the expression 3x+8 and set is equal to 20
so we have
3x+8=20
then subtract the 8 and move it over so we are left with
3x=12
so we divide 12 by three and are left with
x=4