Find the value of side a in the right triangle shown.

It would be 4.9 because you would use a^2+b^2=c^2. The hypotenuse is always c^2. Write this our as 
x^2+5^2=7^2. 
Break these down so its 
x^2+25=49
you want the x alone so you'll subtract 25 from both sides which in the end is 
x^2=24
You want to get rid of the exponent so you place √ in front of 24. In the end it looks like
x=√24 Plug it into a calculator and it'll give you 4.89. Round it and it'll give you answer A (4.9cm)

The length of what is left would be 1 1/2 feet long.