That triangle appears to be an isosceles which means there are two sides that are the same length which would make the the missing angles the same degree and a triangles angles added together should equal 180° so 45+45= 90 and 90+90=180
-10n-3m because the two negatives (-9nand -n) are like terms and they become -10n because the negative n is basically -1n the -3m is there by itself with no other like terms so it stays the same
