Because the two triangles are similar, the angle of one letter would be the same for the corresponding angle of the other triangle.
In the given example R = L
S = M
T = N
Because M is given as 70 degrees, then S is also 70 degrees.
now you have R = 65 and S = 70
The three inside angles of a triangle equal 180, so to find T subtract R ans S from 180.
T = 180 - 65 - 70 = 45
T = 45 degrees.