Solve the problem.

The height of a triangular truss is $5ft$ less than the base. The amount of drywall needed to cover the triangular area is $84f{t}^2.$ Find the base and height of the triangle to the nearest tenth of a foot.

Select one:

a$.$ base $=15\mathrm{.}9ft$; height $=10\mathrm{.}9ft$

b$.$ base $=21ft$; height $=16ft$

c$.$ base $=12ft$; height $=7ft$

d$.$ base $=15\mathrm{.}7ft$; height $=10\mathrm{.}7ft$