Ineed to pass this year use the pythagorean theorem find the missing length a=7 b=11 c= ?

Pythagorean theorem formula is a^2 + b^2 = c^2

7^2 + 11^2 = 170

You will get the value of c by square-rooting 170

Therefore, the answer is √170

There are actually 2 possibilities here:

1) a and b are the shortest 2 sides, and c is the longest side, and we assume that this is a right triangle.  Then 7^2 + 11^2 = c^2,  or  c^2 = 49 + 121 =170.
c is then sqrt(170)   (answer)

2) b is the hypo and a and c are the 2 legs.  Then 11^2 = 7^2 + c^2, and 
121-49 = c^2, so that c = sqrt(72), or sqrt(36)*sqrt(2) = 6*sqrt(2) (answer)