4 more students will need to sign up, because 56 + 4 = 60, and with 60 students there will be 6 teams, each consisting of 10 students. With only 56 students, there would've only been 5 teams, so 4 more.
Remember you can do anything to an equation as long as you do it to both sides
x+9=18-2x add 2x to both sides 2x+x+9=18+2x-2x 3x+9=18+0 3x+9=18 minus 9 both sides 3x+9-9=18-9 3x+0=9 3x=9 divide both sides by 3 (3x)/3=9/3 (3/3)x=3 1x=3 x=3