R1=sym (' R1 ')
R2=sym (' R2)
R3=sym (' R3)
R4 R4=sym (' ')
R5=sym (' R5)
R6=sym (' R6)

Syms R1 R2 and R3 and R4 R5 R6 I1 I2 I3 I4 I5 U U2
F1=sym (' - 20 + R5 - I2 I1 * * R1 - U + R4 * I3==0 ');
F2=sym (' I2 * R1 + R2 I5 * - 10==0 ');
F3=sym (' U + i2 * R3 - I5 * R2==0 ');
F4=sym (' - i2 + 3.5 * * R3 - I3 * R4 R6 - U2==0 ');
F5=sym (' I1 - I3-3.5==0 ');
F6=sym (' I3 - i2-2==0 ');
F7=sym (' I5 - I2-2==0 ');
[I1, I2, I3, I2, I5, U and U2]=solve ([f1, f2, f3, f4 and f5, f6 and f7])
Advising is so of,
The Support of character vectors that are not valid variable names or define a number will be removed in a future release. To create -- expressions, first create --
The variables and then use operations on them.