To find the length of the slanted line, you have to draw a line creating a right triangle. The sides are 17, (81-29=52) and the missing side. You can use the Pythagorean Theorem to find the side:
a2+b2=c2
17^2+52^2=c^2
c^2=2993
c=sqrt2993=54.7, which you can round to 55.
Then to find the perimeter, add up all sides: 17+81+55+29=182. This is the perimeter