The width of a rectangle is 2x yards. The length of the rectangle is 7 yards more

1.   already simplified

2.   foil the first two sets of parenthesis
(3-4i)(1+5i) = 3 + 15i -4i -20i^2 (i^2 = -1 so -20i^2 becomes + 20)
3 + 11i + 20 = 23 + 11i
now subtract (2-i) from 23 + 11i
23 + 11i   - 2 + i = 21 + 12i

3.   c = sqrt (a^2 + b^2)   square both sides
c^2 = a^2 + b^2
c^2 - a^2 = b^2
sqrt(c^2-a^2) = b