Let _ab_ be the line so drawn.
Find _V_ and _V′_, the vanishing-points respectively of the lines _AC_
and _BC_. (Problem III.)
From _a_ draw _aV_, and from _b_, draw _bV′_, cutting each other in
_c_.
Then _abc_ is the triangle required.
If _AC_ is the line originally given, _ac_ is the line which must be
first drawn, and the line _V′b_ must be drawn from _V′_ to _c_ and
produced to cut _ab_ in _b_. Similarly, if _BC_ is given, _Vc_ must be
drawn to _c_ and produced, and _ab_ from its vanishing-point to _b_,
and produced to cut _ac_ in _a_.