Lena's guest house is 15 m long and 12 m wide. how long is the diagonal of the house?

Draw dagarm (see attachment)

we've got a right triangle
the legs are 12 and 15
and the hypohuse is the diagonal

remember, for any right tirnalge
when the legs are a and b and hytponuse is c then
a²+b²=c²

so
12²+15²=c²
144+225=c²
369=c²
sqrt both sides
√369=c
3√41=c
the diagonal is 3√41 m or about 19.2m