so in order to find line AC you must find line AD and DC then plus them together.
to find AD use Pythagoras theorem
a^2 = c^2 - b^2
AD^2 = 7.5^2 - 6.5^2
AD^2 = 56.25 - 42.25
AD^2 = 14
square root both sides to get rid of the ^2
AD ≈ 3.7 or 3.74
Do the same for DC
DC^2 = 10^2 - 6.5^2
DC^2 = 100 - 42.25
DC^2 = 57.75
DC ≈ 7.6
now plus AD and DC which should give u 11.3