Triangle RST is isosceles since sides RS and ST are congruent.
That makes angles SRT and STR congruent.
m<SRT is given as m. m<STR is also m.
Triangle STU is equilateral, so it also equiangular, making m<STU = 60.
m<STR + m<RTU = m<STU
m<RTU is given as n.
m + n = 60