In the diagram below of triangle P, Q, RPQR, SS is the midpoint of start overline, P, R, end overline PR and TT is the midpoint of start overline, Q, R, end overline QR ​ . If S, T, equals, minus, 6, x, plus, 56ST=−6x+56, and P, Q, equals, 10, plus, 5, xPQ=10+5x, what is the measure of start overline, S, T, end overline ST