The triangles are similar.

Notice that

3 * 5  =  15     and     5 * 5  =  25

We can see that the scale factor is  5 .

each side of the larger triangle  =  5 * the corresponding side of the smaller triangle

So....

3x + 8   =   5 * 4

3x + 8  =  20

                            Subtract  8  from both sides.

3x   =   12

                            Divide both sides by  4 .

x   =   4