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Explanation:
For now, focus solely on triangle HGF.
We'll need to find the measure of angle F.
Use the law of cosines
f^2 = g^2 + h^2 - 2*g*h*cos(F)
(4.25)^2 = 8^2 + 6^2 - 2*8*6*cos(F)
18.0625 = 100 - 96*cos(F)
18.0625-100 = -96*cos(F)
-81.9375 = -96*cos(F)
cos(F) = (-81.9375)/(-96)
cos(F) = 0.853515625
F = arccos(0.853515625)
F = 31.403868 degrees approximately
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Now we can move our attention to triangle DEF.
We'll use the angle F we just found to find the length of the opposite side DE, aka side f.
Once again, we use the law of cosines.
f^2 = d^2 + e^2 - 2*d*e*cos(F)
f^2 = (4.75+8)^2 + (11+6)^2 - 2*(4.75+8)*(11+6)*cos(31.403868)
f^2 = 81.563478
f = sqrt(81.563478)
f = 9.031250 approximately
Rounding to two decimal places means we get the final answer of DE = 9.03