help pls quick

Step 1:  Since you know the three sides of the triangle, use the Law of Cosines to find the size of angle(B).

Step 2:  Now, using angle(B), AB and BM, use the Law of Cosines to find AM.

                   A

        13                     15         

B             14                         C

BM =  7

Using  the Law  of  Cosines  we  have

AC^2  =  BC^2 + AB^2 - 2 (BC * AB) cos (ABC)

15^2  = 14^2  + 13^2  - 2(14 * 13) cos (ABC)

[ 15^2  - 14^2  - 13^2 ]  /  [ -2 (14*13) ]  =  cos (ABC)

-140 / -364 =  5/13  =  cos (ABC)

So  AM  can be found as

AM  =  sqrt [ BM^2  + AB^2  -  2(BM * AB) cos (ABC) ]

AM = sqrt  [ 7^2 + 13^2  - 2(7*13) * (5/13) ]

AM = sqrt [ 49 + 169  - 2 * 7 * 5 ]

AM  = sqrt  [ 49 +169 - 70]

AM = sqrt [ 148 ]  =  sqrt [ 4 * 37]  =  2 sqrt [37] units  ≈ 12.166 units