Your problem essentially boils down to solving the followingfourequations simultaneously:

3.5a + 2b = 4 => 7a + 4b = 8 .... (1)

3.5c - 2d = 0.5 => 7c - 4d = 1 .... (2)

a^2 + c^2 = 1 .... (3)

b^2 + d^2 = 1 .... (4)

where all of the a, b, c and d lie between -1 and 1 inclusive.