Landscaper installs 12 sections trellis each section of trellis is 3/4 yards long how many yards of trellis are installed all together

You can write it out as:
12*3/4 or 12/1*3/4

To multiply fractions you can times the top and bottom numbers together, to get the
(12*3)/(1*4) = 36/4

Then you find a common number, a number you can divide the top and bottom fraction by to get a interger (whole number), in this case it is 4. Then you divide both the numerator and denominator by 4:
(36/4)/(4/4) = 9/1

If a fraction has 1 as the bottom number, you can just write out the top number so:
9/1 = 9

Hope this helps! :)