Process: Make AD perpendicular to l3 and CE perpendicular to l3.

It can be proved that △ABD is all equal to △CBE.

Then BD equals CE equals 5.

According to Pythagorean Theorem, AB is equal to the root sign 34.

Because it is an isosceles right triangle, AC equals the root number 34 times the root number 2 and finally equals the root number 17.