Because: |a+b|? =|a|？ +2a*b+|b|？ So: 2a*b=|a+b|? -| One |? -|b|？ Given the modulus of vector A = 13, the modulus of vector B = 19, | A+B | = 24, then: |a-b|? =|a|？ -2a*b+|b|？ =2|a|？ +2|b|？ -|a+b|？ =2×( 13? + 19? )-24? =2×( 169+36 1)-576= 1060-576=484=22? So the solution is |a-b|=22